fix: abstract and fix legacy fs (#4032)

src/constants.ts

@@ -35,6 +35,7 @@ export enum EVENT {
   MOUSE_MOVE = "mousemove",
   RESIZE = "resize",
   UNLOAD = "unload",
+  FOCUS = "focus",
   BLUR = "blur",
   DRAG_OVER = "dragover",
   DROP = "drop",
@@ -84,7 +85,7 @@ export const GRID_SIZE = 20; // TODO make it configurable?
 export const MIME_TYPES = {
   excalidraw: "application/vnd.excalidraw+json",
   excalidrawlib: "application/vnd.excalidrawlib+json",
-};
+} as const;
 
 export const EXPORT_DATA_TYPES = {
   excalidraw: "excalidraw",