feat: show copy-as-png export button on firefox and show steps how to enable it (#6125)

* feat: hide copy-as-png shortcut from help dialog if not supported

* fix: support firefox if clipboard.write supported

* show shrotcut in firefox and instead show error message how to enable the flag support

* widen to TypeError because minification

* show copy-as-png on firefox even if it will throw

src/actions/actionHistory.tsx

@@ -5,10 +5,11 @@ import { t } from "../i18n";
 import History, { HistoryEntry } from "../history";
 import { ExcalidrawElement } from "../element/types";
 import { AppState } from "../types";
-import { isWindows, KEYS } from "../keys";
+import { KEYS } from "../keys";
 import { newElementWith } from "../element/mutateElement";
 import { fixBindingsAfterDeletion } from "../element/binding";
 import { arrayToMap } from "../utils";
+import { isWindows } from "../constants";
 
 const writeData = (
   prevElements: readonly ExcalidrawElement[],