Rollup merge of #96433 - petrochenkov:delim, r=nnethercote

rustc_ast: Harmonize delimiter naming with `proc_macro::Delimiter`

Compiler cannot reuse `proc_macro::Delimiter` directly due to extra impls, but can at least use the same naming.

After this PR the only difference between these two enums is that `proc_macro::Delimiter::None` is turned into `token::Delimiter::Invisible`.
It's my mistake that the invisible delimiter is called `None` on stable, during the stabilization I audited the naming and wrote the docs, but missed the fact that the `None` naming gives a wrong and confusing impression about what this thing is.

cc rust-lang/rust#96421
r? ``@nnethercote``

proc_macro/src/lib.rs

@@ -704,10 +704,10 @@ pub enum Delimiter {
     #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
     Bracket,
     /// `Ø ... Ø`
-    /// An implicit delimiter, that may, for example, appear around tokens coming from a
+    /// An invisible delimiter, that may, for example, appear around tokens coming from a
     /// "macro variable" `$var`. It is important to preserve operator priorities in cases like
     /// `$var * 3` where `$var` is `1 + 2`.
-    /// Implicit delimiters might not survive roundtrip of a token stream through a string.
+    /// Invisible delimiters might not survive roundtrip of a token stream through a string.
     #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
     None,
 }