Update HTMLElement requestPointerLock to return Promise

This is consistent with both the API:
https://developer.mozilla.org/en-US/docs/Web/API/Element/requestPointerLock#return_value

As well as its upcoming inclusion in TypeScript's built-in types for requestPointerLock:
https://github.com/microsoft/TypeScript/blob/90da3ca5dd694f7029b535ea09727be899854d40/src/lib/dom.generated.d.ts#L7873

Which was added here:
microsoft/TypeScript#59259

types/chrome-apps/index.d.ts

@@ -13750,7 +13750,7 @@ interface HTMLElement {
     /**
      * @requires Permissions: 'pointerLock'
      */
-    requestPointerLock(): void;
+    requestPointerLock(): Promise<void>;
     /**
      * @requires Permissions: 'pointerLock'
      */