fix: Emojis and embeds cannot be copied to plain text clipboard (#3561)

shared/typings/prosemirror-model.d.ts

@@ -1,3 +1,4 @@
+import { PlainTextSerializer } from "../editor/types";
 import "prosemirror-model";
 
 declare module "prosemirror-model" {
@@ -7,4 +8,11 @@ declare module "prosemirror-model" {
     // https://github.com/ProseMirror/prosemirror-model/blob/bd13a2329fda39f1c4d09abd8f0db2032bdc8014/src/replace.js#L51
     removeBetween(from: number, to: number): Slice;
   }
+
+  interface NodeSpec {
+    /**
+     * Defines the text representation of the node when copying to clipboard.
+     */
+    toPlainText?: PlainTextSerializer;
+  }
 }