Decouple theme_importer from old GPUI 1 crates (#3866)

This PR decouples the `theme_importer` from the `theme` and `gpui`
crates.

We achieve this by inlining all of the relevant bits needed to
deserialize Zed1 themes.

This will allow us to continue to import Zed1 themes after we land
#3862.

Release Notes:

- N/A

Cargo.lock

@@ -9958,17 +9958,16 @@ dependencies = [
  "anyhow",
  "clap 4.4.4",
  "convert_case 0.6.0",
- "gpui",
  "gpui2",
  "indexmap 1.9.3",
  "json_comments",
  "log",
  "palette",
+ "pathfinder_color",
  "rust-embed",
  "serde",
  "simplelog",
  "strum",
- "theme",
  "theme2",
  "uuid 1.4.1",
 ]