Implement SyncFs based on chrome file api

[rklaehn]

This should be pretty straightforward. Just use the origin private file system. See here: https://web.dev/file-system-access/#accessing-the-origin-private-file-system

The downside of course is that this does not exist on all browsers. But we have the WebCacheFs as a fallback.

https://web.dev/file-system-access/#accessing-the-origin-private-file-system

[rklaehn]

Started working on this in https://github.com/cloudpeers/radixdb/tree/web-file

But there is lots of unstable stuff, so I can see why I came up with the cache hack.