Introduce a new `markdown` crate (#11556)

This pull request introduces a new `markdown` crate which is capable of
parsing and rendering a Markdown source. One of the key additions is
that it enables text selection within a `Markdown` view. Eventually,
this will replace `RichText` but for now the goal is to use it in the
assistant revamped assistant in the spirit of making progress.

<img width="711" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/b56c777b-e57c-42f9-95c1-3ada22f63a69">

Note that this pull request doesn't yet use the new markdown renderer in
`assistant2`. This is because we need to modify the assistant before
slotting in the new renderer and I wanted to merge this independently of
those changes.

Release Notes:

- N/A

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Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Conrad <conrad@zed.dev>
Co-authored-by: Alp <akeles@umd.edu>
Co-authored-by: Zachiah Sawyer <zachiah@proton.me>