Rework `ListItem` and `ListHeader` to use slot-based APIs (#3635)

This PR reworks the `ListItem` and `ListHeader` components to use
slot-based APIs, making them less opinionated about their contents.

Splitting this out of the collab UI styling PR so we can land it to
avoid conflicts.

Co-authored-by: Nate <nate@zed.dev>

Release Notes:

- N/A