Add isSupported function support for language (#17331)

Co-authored-by: fisker Cheung <lionkay@gmail.com>
Co-authored-by: fisker <lionkay@gmail.com>

docs/api.md

@@ -124,6 +124,7 @@ The support information looks like this:
     filenames?: string[];
     linguistLanguageId?: number;
     vscodeLanguageIds?: string[];
+    isSupported?(file: string): boolean;
   }>;
 }
 ```