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/markdown_preview/src/markdown_parser.rs

@@ -1079,9 +1079,7 @@ fn main() {
 
     #[gpui::test]
     async fn test_code_block_with_language(executor: BackgroundExecutor) {
-        let mut language_registry = LanguageRegistry::test();
-        language_registry.set_executor(executor);
-        let language_registry = Arc::new(language_registry);
+        let language_registry = Arc::new(LanguageRegistry::test(executor.clone()));
         language_registry.add(rust_lang());
 
         let parsed = parse_markdown(