Give explicit heights to items in the extension list (#7777)

This PR gives the items in the extension list an explicit height so that
they work properly within the uniform list when descriptions are
missing.

<img width="1235" alt="Screenshot 2024-02-14 at 10 19 14 AM"
src="https://github.com/zed-industries/zed/assets/1486634/01222902-6b05-4e9a-bb5a-bada14b1fd45">

I think we may want to consider using a `list` here instead of a
`uniform_list` to allow them to have variable heights.

Fixes #7756.

Release Notes:

- N/A