This PR adds a theme preview tab to help get an at a glance overview of
the styles in a theme.

You can open it using `debug: open theme preview`.
The next major theme preview PR will move this into it's own crate, as
it will grow substantially as we add content.
Next for theme preview:
- Update layout to two columns, with controls on the right for selecting
theme, layer/elevation-index, etc.
- Cover more UI elements in preview
- Display theme colors in a more helpful way
- Add syntax & markdown previews
Release Notes:
- Added a way to preview the current theme's styles with the `debug:
open theme preview` command.
Similar to https://github.com/zed-industries/zed/pull/18690 &
https://github.com/zed-industries/zed/pull/18695, this PR enables
required docs for `ui` and does some cleanup.
Changes:
- Enables the `deny(missing_docs)` crate-wide.
- Adds `allow(missing_docs)` on many modules until folks pick them up to
document them
- Documents some modules (all in `ui/src/styles`)
- Crate root-level organization: Traits move to `traits`, other misc
organization
- Cleaned out a bunch of unused code.
Note: I'd like to remove `utils/format_distance` but the assistant panel
uses it. To move it over to use the `time_format` crate we may need to
update it to use `time` instead of `chrono`. Needs more investigation.
Release Notes:
- N/A