## What does this PR changed

Adds footnote rendering and navigation to Zed's markdown preview.

- **Footnote references**: (`[^1]`) render inline as `[1]` with link
styling (color + underline)
- **Footnote definitions**: (`[^1]: ...`) render at the bottom with a
horizontal separator, smaller text (85% size), and a label prefix
- **Click-to-navigate**: clicking a footnote reference scrolls to its
definition


https://github.com/user-attachments/assets/a79a0136-f22d-40ac-8b53-cfefa8573d21

## OOS/ Need discussion

- **Display style**: Since currently the gpui crate does not provide a
superscript style, in this PR we publish the feature with using
[`[1]`]() instead of aligning to the GFM styled[^1]
- **Footnote definition placement**: GFM renders the footnote at the
bottom of the content no matter where the user place the footnote
definition, but the `pulldown_cmark` renders the footnote just at where
user place it, for this PR I'll keep the footnote where `pulldown_cmark`
renders it, and we may have some more discuss on if we need to move them
to the bottom of the markdown preview

[^1]: GitHub-flavoured markdown

## What to test

- [ ] Open a markdown file with footnotes (e.g. `Text[^1]\n\n[^1]:
Definition`)
- [ ] Verify reference renders as `[1]` with link color
- [ ] Verify definition renders below a separator with smaller text
- [ ] Verify pointer cursor appears on hover over `[1]`
- [ ] Verify clicking `[1]` scrolls to the definition
- [ ] Verify normal links still work as before
- [ ] `cargo test -p markdown` passes (46 tests)

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #13603

Release Notes:

- Added support for footnotes in Markdown Preview.

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>