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

crates/theme_importer/Cargo.toml

@@ -11,6 +11,7 @@ clap = { version = "4.4", features = ["derive"] }
 convert_case = "0.6.0"
 gpui = { path = "../gpui" }
 indexmap = { version = "1.6.2", features = ["serde"] }
+indoc.workspace = true
 json_comments = "0.2.2"
 log.workspace = true
 palette = { version = "0.7.3", default-features = false, features = ["std"] }