use custom elements for application layout

src/components/app-slot/AppSlotElement.ts

@@ -0,0 +1,14 @@
+import { controller, target } from "@github/catalyst";
+import { AppMainElement } from "components/app-main/AppMainElement";
+import { closest } from "core/utils";
+
+@controller
+class AppSlotElement extends HTMLElement {
+    @target slotElement: HTMLElement;
+    @closest appMain: AppMainElement;
+    constructor() {
+        super();
+    }
+
+    connectedCallback() {}
+}