Add type def for custom reaction type.

src/@types/matrix-js-sdk.d.ts

@@ -0,0 +1,18 @@
+/*
+Copyright 2024 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only
+Please see LICENSE in the repository root for full details.
+*/
+
+import {
+  ElementCallReactionEventType,
+  ECallReactionEventContent,
+} from "../reactions";
+
+// Extend Matrix JS SDK types via Typescript declaration merging to support unspecced event fields and types
+declare module "matrix-js-sdk/src/types" {
+  export interface TimelineEvents {
+    [ElementCallReactionEventType]: ECallReactionEventContent;
+  }
+}