Make copy/cut keyboard shortcuts work in the Discord Activity

- This allows copying to the clipboard to work in the Discord Activity,
but only within JS Paint and between instances of JS Paint, using a text
based clipboard format, and only via Ctrl+C/Ctrl+X (or Cmd+C/Cmd+X).
- Ctrl+V (or Cmd+V) already worked.
- Edit > Cut/Copy/Paste menu items remain nonfunctional.

src/app.js

@@ -1204,7 +1204,7 @@ $G.on("cut copy paste", e => {
 					});
 				}
 			};
-			if (!navigator.clipboard || !navigator.clipboard.write) {
+			if (!navigator.clipboard || !navigator.clipboard.write || is_discord_embed) {
 				return do_sync_clipboard_copy_or_cut();
 			}
 			try {