Decouple theme license generation from TypeScript theme definitions (#3917)

This PR decouples the generation of licenses for the themes we ship from
the TypeScript theme definitions.

For now, we are embedding the license information for the themes in the
`theme_importer`, and emit a combined `LICENSES` file in the `theme`
crate whenever we import themes. This is also where we check that each
theme has a valid license.

We then use this `LICENSES` file when building up the global license
file for Zed.

This decoupling is one step towards us being able to delete the old Zed1
styles.

Release Notes:

- N/A