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>

assets/settings/default.json

@@ -532,6 +532,19 @@
     "enable": false
   },
   "code_actions_on_format": {},
+  // An object whose keys are language names, and whose values
+  // are arrays of filenames or extensions of files that should
+  // use those languages.
+  //
+  // For example, to treat files like `foo.notjs` as JavaScript,
+  // and 'Embargo.lock' as TOML:
+  //
+  // {
+  //   "JavaScript": ["notjs"],
+  //   "TOML": ["Embargo.lock"]
+  // }
+  //
+  "file_types": {},
   // Different settings for specific languages.
   "languages": {
     "Plain Text": {