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/editor/src/inlay_hint_cache.rs

@@ -3387,17 +3387,7 @@ pub mod tests {
         let project = Project::test(fs, ["/a".as_ref()], cx).await;
 
         let language_registry = project.read_with(cx, |project, _| project.languages().clone());
-        language_registry.add(Arc::new(Language::new(
-            LanguageConfig {
-                name: "Rust".into(),
-                matcher: LanguageMatcher {
-                    path_suffixes: vec!["rs".to_string()],
-                    ..Default::default()
-                },
-                ..Default::default()
-            },
-            Some(tree_sitter_rust::language()),
-        )));
+        language_registry.add(crate::editor_tests::rust_lang());
         let mut fake_servers = language_registry.register_fake_lsp_adapter(
             "Rust",
             FakeLspAdapter {