Add initial markdown preview to Zed (#6958)

Adds a "markdown: open preview" action to open a markdown preview.

https://github.com/zed-industries/zed/assets/18583882/6fd7f009-53f7-4f98-84ea-7dd3f0dd11bf


This PR extends the work done in `crates/rich_text` to render markdown
to also support:

- Variable heading sizes
- Markdown tables
- Code blocks
- Block quotes

## Release Notes

- Added `Markdown: Open preview` action to partially close
([#6789](https://github.com/zed-industries/zed/issues/6789)).

## Known issues that will not be included in this PR

- Images.
- Nested block quotes.
- Footnote Reference.
- Headers highlighting.
- Inline code highlighting (this will need to be implemented in
`rich_text`)
- Checkboxes (`- [ ]` and `- [x]`)
- Syntax highlighting in code blocks.
- Markdown table text alignment.
- Inner markdown URL clicks