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/extension/src/extension_store_test.rs

@@ -249,7 +249,7 @@ async fn test_extension_store(cx: &mut TestAppContext) {
         .collect(),
     };
 
-    let language_registry = Arc::new(LanguageRegistry::test());
+    let language_registry = Arc::new(LanguageRegistry::test(cx.executor()));
     let theme_registry = Arc::new(ThemeRegistry::new(Box::new(())));
     let node_runtime = FakeNodeRuntime::new();