[Markdown] Fix backticks and code-spans in table cells

[deathaxe]

Fixes #3968

This commit...

1. removes invalid.deprecated scope from single backticks in table cells
2. fixes code spans treatment in tables. Codespans require balanced backticks in a single line not containing a | cell separator are found.

[deathaxe]

The only downside of current implementation - theoretically - is template syntaxes such as Astro not being able to inject temblate tags into code spans in table cells.

| column 1  | column 2
| ---       | ---
| `{{key}}` | `{{value}}`

Those would currently appear as normal raw text.

[keith-hall]

The only downside of current implementation - theoretically - is template syntaxes such as Astro not being able to inject temblate tags into code spans in table cells.

| column 1  | column 2
| ---       | ---
| `{{key}}` | `{{value}}`

Those would currently appear as normal raw text.

If that will really be a problem then I imagine it could be fixable by having the whole table-cell-code-spans use a lookahead and push into a context. If the lookahead handles the balanced backticks, the inner context can just pop when it sees the closing backticks and consume them all without worrying about how many there are. Edit: but that only works if it's not allowed to have an opening backtick immediately after a closing backtick...