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