fix(overlays): move prepareOverlay to connectedCallback

For custom elements builds, overlays cannot use hasAttribute() in the constructor, so moving it to connectedCallback instead.

core/src/components/toast/toast.tsx

@@ -126,7 +126,7 @@ export class Toast implements ComponentInterface, OverlayInterface {
    */
   @Event({ eventName: 'ionToastDidDismiss' }) didDismiss!: EventEmitter<OverlayEventDetail>;
 
-  constructor() {
+  connectedCallback() {
     prepareOverlay(this.el);
   }