Follow-up to https://github.com/zed-industries/zed/pull/53290

This PR fixes a mistake I pushed before of making the `ListItem`'s
height method take pixels instead of a scalable unit like rems. Now, it
takes `DefiniteLength` which can house both values, meaning we should be
clear to set an explicit height for all of these items while still
preserving font-size scaling.

Release Notes:

- N/A