fix(menu): declare popover property type on HTMLElement

Without it the popover property got renamed by closure.

PiperOrigin-RevId: 584184281

Author: material-web-copybara

menu/internal/controllers/surfacePositionController.ts

@@ -8,7 +8,7 @@ import {ReactiveController, ReactiveControllerHost} from 'lit';
 import {StyleInfo} from 'lit/directives/style-map.js';
 
 /**
- * Declare popoverAPI functions. See
+ * Declare popoverAPI functions and properties. See
  * https://developer.mozilla.org/en-US/docs/Web/API/Popover_API
  * Without this, closure will rename these functions. Can remove once these
  * functions make it into the typescript lib.
@@ -18,6 +18,7 @@ declare global {
     showPopover(): void;
     hidePopover(): void;
     togglePopover(force: boolean): void;
+    popover: string | null;
   }
 }