This PR introduces a markdown preview max-width setting in which we
allow to limit by a given pixel number how wide the content within the
preview tab should go. Before this, content would render edge-to-edge
and if you're reading long markdown docs with no split panes, having it
be full width like that was a horrible reading experience.

So, right now, you can control that through the
`zed://settings/markdown_preview.max_width` setting.

<img width="600" alt="Screenshot 2026-06-18 at 1  24@2x"
src="https://github.com/user-attachments/assets/7f0216e1-e4ac-47f7-975b-5de7a0dd4c6d"
/>

Release Notes:

- Added setting to control max-width of content within the markdown
preview.