Move MediaElement interface to the global types file

src/@types/global.d.ts

@@ -21,4 +21,10 @@ declare global {
     // TODO: https://gitlab.matrix.org/matrix-org/olm/-/issues/10
     OLM_OPTIONS: Record<string, string>;
   }
+
+  // TypeScript doesn't know about the experimental setSinkId method, so we
+  // declare it ourselves
+  interface MediaElement extends HTMLMediaElement {
+    setSinkId: (id: string) => void;
+  }
 }