Add a setting for custom associations between languages and files (#9290)

Closes #5178

Release Notes:

- Added a `file_types` setting that can be used to associate languages
with file names and file extensions. For example, to interpret all `.c`
files as C++, and files called `MyLockFile` as TOML, add the following
to `settings.json`:

    ```json
    {
      "file_types": {
        "C++": ["c"],
        "TOML": ["MyLockFile"]
      }
    }
    ```

As with most zed settings, this can be configured on a per-directory
basis by including a local `.zed/settings.json` file in that directory.

---------

Co-authored-by: Marshall <marshall@zed.dev>

crates/workspace/src/workspace.rs

@@ -405,7 +405,7 @@ impl AppState {
         }
 
         let fs = fs::FakeFs::new(cx.background_executor().clone());
-        let languages = Arc::new(LanguageRegistry::test());
+        let languages = Arc::new(LanguageRegistry::test(cx.background_executor().clone()));
         let clock = Arc::new(clock::FakeSystemClock::default());
         let http_client = util::http::FakeHttpClient::with_404_response();
         let client = Client::new(clock, http_client.clone(), cx);