feat: Add OtherItemsProxy TouchBar item (#22270)

* feat: Add OtherItemsProxy touchbar item

* review!

lib/browser/api/touch-bar.js

@@ -51,13 +51,27 @@ class TouchBar extends EventEmitter {
         item.child.ordereredItems.forEach(registerItem)
       }
     }
+
+    let hasOtherItemsProxy = false
     items.forEach((item) => {
       if (!(item instanceof TouchBarItem)) {
         throw new Error('Each item must be an instance of TouchBarItem')
       }
+
+      if (item.type === 'other_items_proxy') {
+        if (!hasOtherItemsProxy) {
+          hasOtherItemsProxy = true
+        } else {
+          throw new Error('Must only have one OtherItemsProxy per TouchBar')
+        }
+      }
+    })
+
+    // register in separate loop after all items are validated
+    for (const item of items) {
       this.ordereredItems.push(item)
       registerItem(item)
-    })
+    }
   }
 
   set escapeItem (item) {
@@ -334,4 +348,11 @@ TouchBar.TouchBarScrubber = class TouchBarScrubber extends TouchBarItem {
   }
 }
 
+TouchBar.TouchBarOtherItemsProxy = class TouchBarOtherItemsProxy extends TouchBarItem {
+  constructor (config) {
+    super()
+    this._addImmutableProperty('type', 'other_items_proxy')
+  }
+}
+
 module.exports = TouchBar