Factor out collaboration code (#2313)

Co-authored-by: Lipis <lipiridis@gmail.com>
Co-authored-by: dwelle <luzar.david@gmail.com>

src/excalidraw-app/app_constants.ts

@@ -0,0 +1,14 @@
+// time constants (ms)
+export const SAVE_TO_LOCAL_STORAGE_TIMEOUT = 300;
+export const INITIAL_SCENE_UPDATE_TIMEOUT = 5000;
+export const SYNC_FULL_SCENE_INTERVAL_MS = 20000;
+
+export const BROADCAST = {
+  SERVER_VOLATILE: "server-volatile-broadcast",
+  SERVER: "server-broadcast",
+};
+
+export enum SCENE {
+  INIT = "SCENE_INIT",
+  UPDATE = "SCENE_UPDATE",
+}