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:
18431051d9/crates/language2/src/highlight_map.rs (L15-L41)
