Remove static phrasing content

Previously, there was a difference between static and dynamic phrasing
content.
There was also transparent content, to “inherit” the (static or dynamic)
content model from a parent.
This was used to prohibit links in links.

However, while it is unwise to put links in links, it is possible with
markdown to do so, by embedding autolinks in links with resources or
references.
Take for example:

```markdown
[alpha <https://bravo> charlie](delta)
```

Yields, per CommonMark:

```html
<p><a href="delta">alpha <a href="https://bravo">https://bravo</a> charlie</a></p>
```

See also: commonmark/commonmark-spec#719.

There is also the case where while it is unwise to author such markdown,
tools transforming mdast might inject links.
It’s probably good to handle such cases in `mdast-util-to-markdown` and
such?

Not allowing links in links is also something imposed by HTML (particularly
the parsing side[^1]): perhaps mdast could be used in places where this
restriction doesn’t need to be imposed.

Finally, it is hard to type such inherited content models.
As in, transparent content is not used in `@types/mdast` (I don’t think it
can be typed?):

https://github.com/DefinitelyTyped/DefinitelyTyped/blob/3b052a3e5b59b0efaa22669d9bd8f268bd689835/types/mdast/index.d.ts#L286

So, this PR is a proposal to remove the difference and simplify the AST.

Closes GH-42.

[^1]: You can produce nested `a`s with the DOM:

    ```js
    let a1 = document.createElement('a'); a1.textContent = 'alpha';
    let a2 = document.createElement('a'); a2.textContent = 'bravo';
    a1.appendChild(a2)
    document.body.appendChild(a1)
    ```

readme.md

@@ -53,8 +53,6 @@ The latest released version is [`4.0.0`][latest].
     *   [`Content`](#content)
     *   [`ListContent`](#listcontent)
     *   [`PhrasingContent`](#phrasingcontent)
-    *   [`StaticPhrasingContent`](#staticphrasingcontent)
-    *   [`TransparentContent`](#transparentcontent)
 *   [Extensions](#extensions)
     *   [GFM](#gfm)
     *   [Frontmatter](#frontmatter)
@@ -527,7 +525,7 @@ Yields:
 ```idl
 interface Emphasis <: Parent {
   type: 'emphasis'
-  children: [TransparentContent]
+  children: [PhrasingContent]
 }
 ```
 
@@ -536,7 +534,7 @@ contents.
 
 **Emphasis** can be used where [**phrasing**][dfn-phrasing-content] content is
 expected.
-Its content model is [**transparent**][dfn-transparent-content] content.
+Its content model is [**phrasing**][dfn-phrasing-content] content.
 
 For example, the following markdown:
 
@@ -568,7 +566,7 @@ Yields:
 ```idl
 interface Strong <: Parent {
   type: 'strong'
-  children: [TransparentContent]
+  children: [PhrasingContent]
 }
 ```
 
@@ -577,7 +575,7 @@ or urgency for its contents.
 
 **Strong** can be used where [**phrasing**][dfn-phrasing-content] content is
 expected.
-Its content model is [**transparent**][dfn-transparent-content] content.
+Its content model is [**phrasing**][dfn-phrasing-content] content.
 
 For example, the following markdown:
 
@@ -674,7 +672,7 @@ Yields:
 ```idl
 interface Link <: Parent {
   type: 'link'
-  children: [StaticPhrasingContent]
+  children: [PhrasingContent]
 }
 
 Link includes Resource
@@ -684,7 +682,7 @@ Link includes Resource
 
 **Link** can be used where [**phrasing**][dfn-phrasing-content] content is
 expected.
-Its content model is [**static phrasing**][dfn-static-phrasing-content] content.
+Its content model is also [**phrasing**][dfn-phrasing-content] content.
 
 **Link** includes the mixin [**Resource**][dfn-mxn-resource].
 
@@ -747,7 +745,7 @@ Yields:
 ```idl
 interface LinkReference <: Parent {
   type: 'linkReference'
-  children: [StaticPhrasingContent]
+  children: [PhrasingContent]
 }
 
 LinkReference includes Reference
@@ -758,7 +756,7 @@ association, or its original source if there is no association.
 
 **LinkReference** can be used where [**phrasing**][dfn-phrasing-content] content
 is expected.
-Its content model is [**static phrasing**][dfn-static-phrasing-content] content.
+Its content model is also [**phrasing**][dfn-phrasing-content] content.
 
 **LinkReference** includes the mixin [**Reference**][dfn-mxn-reference].
 
@@ -961,27 +959,12 @@ type ListContent = ListItem
 ### `PhrasingContent`
 
 ```idl
-type PhrasingContent = Link | LinkReference | StaticPhrasingContent
+type PhrasingContent = Break | Emphasis | HTML | Image | ImageReference
+  | InlineCode | Link | LinkReference | Strong | Text
 ```
 
 **Phrasing** content represent the text in a document, and its markup.
 
-### `StaticPhrasingContent`
-
-```idl
-type StaticPhrasingContent =
-  Break | Emphasis | HTML | Image | ImageReference | InlineCode | Strong | Text
-```
-
-**StaticPhrasing** content represent the text in a document, and its
-markup, that is not intended for user interaction.
-
-### `TransparentContent`
-
-The **transparent** content model is derived from the content model of its
-[parent][dfn-parent].
-Effectively, this is used to prohibit nested links (and link references).
-
 ## Extensions
 
 Markdown syntax is often extended.
@@ -1196,7 +1179,7 @@ or indeterminate or not applicable (when `null` or not present).
 ```idl
 interface Delete <: Parent {
   type: 'delete'
-  children: [TransparentContent]
+  children: [PhrasingContent]
 }
 ```
 
@@ -1205,7 +1188,7 @@ accurate or no longer relevant.
 
 **Delete** can be used where [**phrasing**][dfn-phrasing-content] content is
 expected.
-Its content model is [**transparent**][dfn-transparent-content] content.
+Its content model is [**phrasing**][dfn-phrasing-content] content.
 
 For example, the following markdown:
 
@@ -1269,11 +1252,10 @@ type RowContent = TableCell
 type ListContentGfm = ListItemGfm
 ```
 
-#### `StaticPhrasingContent` (GFM)
+#### `PhrasingContent` (GFM)
 
 ```idl
-type StaticPhrasingContentGfm =
-  FootnoteReference | Delete | StaticPhrasingContent
+type PhrasingContentGfm = FootnoteReference | Delete | PhrasingContent
 ```
 
 ### Frontmatter
@@ -1365,10 +1347,10 @@ Yields:
 }
 ```
 
-#### `StaticPhrasingContent` (footnotes)
+#### `PhrasingContent` (footnotes)
 
 ```idl
-type StaticPhrasingContentFootnotes = Footnote | StaticPhrasingContent
+type PhrasingContentFootnotes = Footnote | PhrasingContent
 ```
 
 ### MDX
@@ -1671,10 +1653,6 @@ projects!
 
 [dfn-phrasing-content]: #phrasingcontent
 
-[dfn-static-phrasing-content]: #staticphrasingcontent
-
-[dfn-transparent-content]: #transparentcontent
-
 [gfm-section]: #gfm
 
 [gfm-footnote]: https://github.blog/changelog/2021-09-30-footnotes-now-supported-in-markdown-fields/