Extract `SyntaxTheme` into its own lightweight crate so that downstream
consumers can use syntax highlighting colors without pulling in the full
`theme` crate and its transitive dependencies.

## Changes

**Commit 1 — Extract SyntaxTheme into its own crate**

Move `SyntaxTheme`, `SyntaxThemeSettings`, `HighlightStyle`, and
supporting types from `theme/src/styles/syntax.rs` into a new
`syntax_theme` crate that depends only on `gpui`. The `theme` crate
re-exports everything for backward compatibility — no call-site changes
needed.

**Commit 2 — Add `bundled-themes` feature with One Dark**

Add an optional `bundled-themes` feature that bundles `one_dark()`, a
`SyntaxTheme` loaded from the existing One Dark JSON theme file. This
lets consumers get a usable syntax theme without depending on the full
theme machinery.

Release Notes:

- N/A