chore: revert outlet removal since needed for local backdrop

packages/overlays/docs/demo-overlay-system.js

@@ -40,6 +40,7 @@ class DemoOverlaySystem extends OverlayMixin(LitElement) {
   render() {
     return html`
       <slot name="invoker"></slot>
+      <slot name="_overlay-shadow-outlet"></slot>
       <div id="overlay-content-node-wrapper">
         <slot name="content"></slot>
       </div>

packages/overlays/src/OverlayController.js

@@ -895,6 +895,7 @@ export class OverlayController extends EventTargetShim {
             /** @type {HTMLElement} */
             (this.backdropNode).classList.add('local-overlays__backdrop');
           }
+          this.backdropNode.slot = '_overlay-shadow-outlet';
           /** @type {HTMLElement} */
           (this.contentNode.parentNode).insertBefore(this.backdropNode, this.contentNode);
           break;