feat: Add a `subtype?: string` property to `ExcalidrawElement`.

src/element/types.ts

@@ -65,6 +65,7 @@ type _ExcalidrawElementBase = Readonly<{
   updated: number;
   link: string | null;
   locked: boolean;
+  subtype?: string;
   customData?: Record<string, any>;
 }>;