Add offline indicator (also to top zone example) (#2083)

This PR adds an offline indicator to the UI package. It's not used in
the default app but we'll use it on tldraw.com, and it makes sense to
include it here as it's generally useful.

### Change Type

- [x] `minor` — New feature

### Test Plan

1. See the zones example.

### Release Notes

- [@tldraw/tldraw] add offline indicator to ui components

apps/examples/src/examples/ZonesExample.tsx

@@ -1,10 +1,10 @@
-import { Tldraw } from '@tldraw/tldraw'
+import { OfflineIndicator, Tldraw } from '@tldraw/tldraw'
 import '@tldraw/tldraw/tldraw.css'
 
 export default function Example() {
 	return (
 		<div className="tldraw__editor">
-			<Tldraw shareZone={<CustomShareZone />} />
+			<Tldraw topZone={<OfflineIndicator />} shareZone={<CustomShareZone />} />
 		</div>
 	)
 }