Add button when scrolled outside of visible area (#643)

With the infinite scroll behavior, it's easy to scroll super far away from where the content is and have a hard time getting back. This PR adds a button to refocus on the center of the scene when no elements are visible anymore.

src/types.ts

@@ -22,6 +22,7 @@ export type AppState = {
   scrollY: number;
   cursorX: number;
   cursorY: number;
+  scrolledOutside: boolean;
   name: string;
   selectedId?: string;
 };