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.
a347c4def7