Unable to get file size of local zarr files

[kcpevey]

The napari-ome-zarr plugin doesn't store the path to a local zarr file in layer.source.path so once its loaded, we have no way of knowing the original path. Therefore, its not possible to get the local file size on disk for zarr files at this time.

If this is resolved in the upstream package, we have a method, napari_metadata._file_size.directory_size which can be used to determine the size of zarr files (which are actually a directory of files).