feat: add onBeforeMount callback to createIframeUi options (#1899)

Co-authored-by: Aaron <aaronklinker1@gmail.com>

packages/wxt/src/utils/content-script-ui/iframe.ts

@@ -22,6 +22,7 @@ export function createIframeUi<TMounted>(
   let mounted: TMounted | undefined = undefined;
   const mount = () => {
     applyPosition(wrapper, iframe, options);
+    options.onBeforeMount?.(wrapper, iframe);
     mountUi(wrapper, options);
     mounted = options.onMount?.(wrapper, iframe);
   };
@@ -73,4 +74,9 @@ export type IframeContentScriptUiOptions<TMounted> =
      * Optionally return a value that can be accessed at `ui.mounted` or in the `onRemove` callback.
      */
     onMount?: (wrapper: HTMLElement, iframe: HTMLIFrameElement) => TMounted;
+    /**
+     * Callback executed before mounting the UI. Use this function to customize the iframe or wrapper
+     * elements before they are injected into the DOM. It is called every time `ui.mount()` is called.
+     */
+    onBeforeMount?: (wrapper: HTMLElement, iframe: HTMLIFrameElement) => void;
   };