Closes #44306

This PR makes two changes:
- Uses the new `grid_cols_min_content` API. See more here:
https://github.com/zed-industries/zed/pull/44973.
- Changes Markdown table rendering to use a single grid instead of
creating a new grid per row, so column widths stay consistent across
rows.

Release Notes:

- Fixed an issue where Markdown tables wouldn't render in the hover
popover.