Add syntax scopes to themes (#25323)

Release Notes:

  - Added syntax scopes to themes

Supports:

- [Improve CSS syntax
highlighting](https://github.com/zed-industries/zed/pull/25326)
- [Improve Go syntax
highlighting](https://github.com/zed-industries/zed/pull/25327)
- [Improve Markdown syntax
highlighting](https://github.com/zed-industries/zed/pull/25330)

Changes:

- Adds highlighting rules for the following new scopes, using theme
colors:
    - `heading`
    - `namespace`
    - `selector`
    - `strikethrough`
    - `unit`

- Renames scopes that are no longer used in `zed/crates/languages/src`
or `zed/extensions` to their new names:
    - `punctuation.list_marker` -> `punctuation.markup`
    - `link_text` -> `link`
- `link_uri` -> `link.url`, as defined in the [gitcommit
grammar](https://github.com/zed-industries/zed/blob/dff47a843695d03160680502e6d94634e376698e/crates/languages/src/gitcommit/highlights.scm#L5)
    - `text.literal` -> `raw`