Implement Save without re-prompt and Save as (#1709)

* Implement Save without re-prompt and Save as
Fixes #1668

* Add save-as icon

* Make .excalidraw the default extension

* Only show save as button on supporting browsers

src/components/LayerUI.tsx

@@ -130,6 +130,7 @@ const LayerUI = ({
           <Stack.Row gap={1} justifyContent="space-between">
             {actionManager.renderAction("loadScene")}
             {actionManager.renderAction("saveScene")}
+            {actionManager.renderAction("saveAsScene")}
             {renderExportDialog()}
             {actionManager.renderAction("clearCanvas")}
             <RoomDialog