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/appState.ts

@@ -24,6 +24,7 @@ export function getDefaultAppState(): AppState {
     scrollY: 0,
     cursorX: 0,
     cursorY: 0,
+    scrolledOutside: false,
     name: DEFAULT_PROJECT_NAME,
   };
 }