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>

docs/src/configuring_zed.md

@@ -380,6 +380,25 @@ To override settings for a language, add an entry for that language server's nam
 
 `boolean` values
 
+## File Types
+
+- Setting: `file_types`
+- Description: Configure how Zed selects a language for a file based on its filename or extension.
+- Default: `{}`
+
+**Examples**
+
+To interpret all `.c` files as C++, and files called `MyLockFile` as TOML:
+
+```json
+{
+  "file_types": {
+    "C++": ["c"],
+    "TOML": ["MyLockFile"]
+  }
+}
+```
+
 ## Git
 
 - Description: Configuration for git-related features.