message: improve typing in the idb worker
diff: @@ -16,6 +16,8 @@ limitations under the License.
  import { IndexedDBStoreWorker } from "matrix-js-sdk/src/indexeddb-worker";
  -const remoteWorker = new IndexedDBStoreWorker(postMessage as InstanceType<typeof Worker>["postMessage"]);
  +const ctx: Worker = self as any;
  -global.onmessage = remoteWorker.onMessage;
  +const remoteWorker = new IndexedDBStoreWorker(ctx.postMessage);
  +
  +ctx.onmessage = remoteWorker.onMessage;
  