fix(api): `WebviewWindow` constructor is public (#1888)

.changes/webview-window-constructor-pub.md

@@ -0,0 +1,5 @@
+---
+"api": patch
+---
+
+Mark the `WebviewWindow` constructor as public.

tooling/api/src/window.ts

@@ -249,7 +249,7 @@ class WebviewWindowHandle {
  * ```
  */
 class WebviewWindow extends WebviewWindowHandle {
-  private constructor(label: string, options: WindowOptions = {}) {
+  constructor(label: string, options: WindowOptions = {}) {
     super(label)
     invokeTauriCommand({
       __tauriModule: 'Window',