Add support for background colors on fenced code blocks in Markdown

[scfurl]

#7336

Self-Review Checklist:

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

Closes #7336 (Technically it was already closed, but this solves the issue which was punted before closing)

Replaces #53180 (which was opened from my fork's main branch and included unrelated commits).

Release Notes:

• Added full-width background color support for markdown fenced code blocks,
  configurable via text.literal.block.markup in theme syntax overrides
• On the left is the current Zed build, on the right is the new build after incorporating code changes