Remove old Theme definition (#3195)

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

Cargo.toml

@@ -92,7 +92,6 @@ members = [
     "crates/text",
     "crates/theme",
     "crates/theme2",
-    "crates/theme_converter",
     "crates/theme_selector",
     "crates/ui2",
     "crates/util",