Add customisation point for declaring widget variables

[turt2live]

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.

---

Tested with the following implementation:

class TestImpl implements IWidgetVariablesCustomisations {
    private promise = new LazyValue<void>(() => new Promise<void>(resolve => {
        setTimeout(() => {
            console.log("@@ RESOLVE");
            resolve();
        }, 10000);
    }));

    provideVariables(): Partial<Omit<ITemplateParams, "widgetRoomId">> {
        return {
            userDisplayName: "OVERRIDE",
        };
    }

    isReady(): Promise<void> {
        return this.promise.value;
    }
}

---

Notes: Add customisation point to override widget variables
element-web notes: none

[jryans]

Hmm, I can imagine other use cases might need the customisation to know more about the widget via an argument, but anyway that can be added later if needed.

[turt2live]

Maybe we can fix that when we take out the words "stop gap". Though optimism is wearing thin there...