Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] 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 #10022 

Release Notes:

- Added support for GFM alert callouts (`> [!NOTE]`, `> [!TIP]`, `>
[!IMPORTANT]`, `> [!WARNING]`, `> [!CAUTION]`) in markdown preview,
rendering each type with a colored left border, icon, and bold label.

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Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>