feat: introduce frames (#6123)

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

src/element/typeChecks.ts

@@ -12,6 +12,7 @@ import {
   ExcalidrawImageElement,
   ExcalidrawTextElementWithContainer,
   ExcalidrawTextContainer,
+  ExcalidrawFrameElement,
   RoundnessType,
 } from "./types";
 
@@ -45,6 +46,12 @@ export const isTextElement = (
   return element != null && element.type === "text";
 };
 
+export const isFrameElement = (
+  element: ExcalidrawElement | null,
+): element is ExcalidrawFrameElement => {
+  return element != null && element.type === "frame";
+};
+
 export const isFreeDrawElement = (
   element?: ExcalidrawElement | null,
 ): element is ExcalidrawFreeDrawElement => {