languages: Add syntax highlighting for HTML character references (#48629)

HTML character references like `·`, `'`, and `{` are
correctly parsed by tree-sitter as named nodes
(`html_character_reference` in TSX/JavaScript, `entity` in HTML), but no
highlight query captures them. This means they render as plain,
unhighlighted text in the editor.

This PR adds one-line highlight captures for each:

- **TSX** (`crates/languages/src/tsx/highlights.scm`):
`(html_character_reference) @string.special`
- **JavaScript** (`crates/languages/src/javascript/highlights.scm`):
`(html_character_reference) @string.special`
- **HTML** (`extensions/html/languages/html/highlights.scm`): `(entity)
@string.special`

`@string.special` is already styled by all built-in themes (One Dark,
Ayu, Gruvbox, etc.), so no theme changes are needed.

Release Notes:

- Added syntax highlighting for HTML character references (`·`,
`'`, `{`, etc.) in TSX, JavaScript, and HTML files.

extensions/html/languages/html/highlights.scm

@@ -7,6 +7,7 @@
   (attribute_value)
 ] @string
 (comment) @comment
+(entity) @string.special
 
 "=" @punctuation.delimiter.html