Add customisation point for declaring widget variables

Fixes element-hq/element-web#18035 (indirectly; issue is on a non-standard version of Element).

This has an isReady() step because the variable usage code path cannot be made async, so instead we let the async loading state happen in the prepare() stage. This is required for the intended usecase.

src/stores/widgets/StopGapWidget.ts

@@ -32,8 +32,11 @@ import {
     IWidgetApiErrorResponseData,
     WidgetKind,
 } from "matrix-widget-api";
-import { StopGapWidgetDriver } from "./StopGapWidgetDriver";
 import { EventEmitter } from "events";
+import { MatrixEvent } from "matrix-js-sdk/src/models/event";
+import { logger } from "matrix-js-sdk/src/logger";
+
+import { StopGapWidgetDriver } from "./StopGapWidgetDriver";
 import { WidgetMessagingStore } from "./WidgetMessagingStore";
 import RoomViewStore from "../RoomViewStore";
 import { MatrixClientPeg } from "../../MatrixClientPeg";
@@ -52,14 +55,11 @@ import ThemeWatcher from "../../settings/watchers/ThemeWatcher";
 import { getCustomTheme } from "../../theme";
 import CountlyAnalytics from "../../CountlyAnalytics";
 import { ElementWidgetCapabilities } from "./ElementWidgetCapabilities";
-import { MatrixEvent } from "matrix-js-sdk/src/models/event";
 import { ELEMENT_CLIENT_ID } from "../../identifiers";
 import { getUserLanguage } from "../../languageHandler";
 import { WidgetVariableCustomisations } from "../../customisations/WidgetVariables";
 import { arrayFastClone } from "../../utils/arrays";
 
-import { logger } from "matrix-js-sdk/src/logger";
-
 // TODO: Destroy all of this code
 
 interface IAppTileProps {