Export TimestampToEventResponse to use in matrix-react-sdk tests (#3223)

* Export ITimestampToEventResponse to use in matrix-react-sdk tests

Part of matrix-org/matrix-react-sdk#10405

* Remove I from interface

As suggested by @weeman1337,
#3223 (review)

See code style guide, https://github.com/vector-im/element-web/blob/50f8be4a623d2280a72920eb1679a4754961f807/code_style.md#typescript--javascript-typescript-javascript

> Interface names should not be marked with an uppercase `I`.

Author: MadLittleMods

src/client.ts

@@ -837,7 +837,7 @@ interface IRoomHierarchy {
     next_batch?: string;
 }
 
-interface ITimestampToEventResponse {
+export interface TimestampToEventResponse {
     event_id: string;
     origin_server_ts: string;
 }
@@ -9541,7 +9541,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
         roomId: string,
         timestamp: number,
         dir: Direction,
-    ): Promise<ITimestampToEventResponse> {
+    ): Promise<TimestampToEventResponse> {
         const path = utils.encodeUri("/rooms/$roomId/timestamp_to_event", {
             $roomId: roomId,
         });