[fs/local] read directory entries until depleted

issue #400

from
https://developer.mozilla.org/en-US/docs/Web/API/DirectoryEntry#createReader

> Continue calling readEntries() until an empty array is returned. You
> have to do this because the API might not return all entries in a
> single call.

I was getting entries in 100-element slices, that's why any directory
with more files was not scanned completely