Load JSON themes (#6893)

This PR changes the theme loading to use the JSON themes bundled with
the binary rather then the Rust theme definitions.

### Performance

I profiled this using `cargo run --release` to see what the speed
differences would be now that we're deserializing JSON:

**Before:** `ThemeRegistry::load_user_themes` took 16.656666ms
**After:** `ThemeRegistry::load_user_themes` took 18.784875ms

It's slightly slower, but not by much. There is probably some work we
could do here to bring down the theme loading time in general.

Release Notes:

- N/A

script/generate-licenses

@@ -9,7 +9,7 @@ OUTPUT_FILE=$(pwd)/assets/licenses.md
 echo -e "# ###### THEME LICENSES ######\n" >> $OUTPUT_FILE
 
 echo "Generating theme licenses"
-cat crates/theme/src/themes/LICENSES >> $OUTPUT_FILE
+cat assets/themes/LICENSES >> $OUTPUT_FILE
 
 echo -e "# ###### CODE LICENSES ######\n" >> $OUTPUT_FILE