feat: add element.updated (#4070)

src/element/types.ts

@@ -45,6 +45,8 @@ type _ExcalidrawElementBase = Readonly<{
   groupIds: readonly GroupId[];
   /** Ids of (linear) elements that are bound to this element. */
   boundElementIds: readonly ExcalidrawLinearElement["id"][] | null;
+  /** epoch (ms) timestamp of last element update */
+  updated: number;
 }>;
 
 export type ExcalidrawSelectionElement = _ExcalidrawElementBase & {