This PR removes the old `Theme` definition in favor of the new
`ThemeVariant`s.

The new `SyntaxStyles` have been reverted to the old `SyntaxTheme` that
operates by storing the syntax styles as a vector of
`gpui2::HighlightStyle`s.

This is necessary for the intended usage by `language2`, where we find
the longest key in the theme's syntax styles that matches the capture
name:

https://github.com/zed-industries/zed/blob/18431051d9d750d9e66284a71f7a55a1e31c1374/crates/language2/src/highlight_map.rs#L15-L41