refac: isolate disposable interface to new_tab test utility

Author: suveshmoza

@types/gecko.d.mts

@@ -253,8 +253,6 @@ declare var MozElements: MozElements;
 
 interface BrowserTab extends MozTabbrowserTab {
   _glide_commandline: GlideCommandLine | null;
-  [Symbol.dispose](): void;
-  [Symbol.asyncDispose](): Promise<void>;
 }
 // Source browser/components/tabbrowser/content/tab.js
 interface MozTabbrowserTab extends XULElement {

src/glide/browser/base/content/GlideTestUtils.sys.mts

@@ -50,7 +50,7 @@ class GlideTestUtilsClass {
    * Behaves the same as `BrowserTestUtils.openNewForegroundTab()` but also defines
    * `[Symbol.dispose]()` so you can use it with `using`.
    */
-  async new_tab(uri?: string): Promise<BrowserTab> {
+  async new_tab(uri?: string): Promise<BrowserTab & Disposable> {
     const tab = await g.BrowserTestUtils.openNewForegroundTab(g.gBrowser, uri);
     return Object.assign(tab, {
       [Symbol.dispose]() {