prefix should prepended to items returned by list_prefix and list_dir methods

[grlee77]

Based on examples in the v3 spec, the prefix used as an argument to FileSystemStore's list_prefix and list_dir should show up in the output.

Example :

import zarrita
h = zarrita.get_hierarchy('../zarr_implementations/data/zarrita.zr3/')
h.store.list_prefix('meta/root/')

gives (after this PR):

['meta/root/blosc/lz4.array.json',
 'meta/root/gzip.array.json',
 'meta/root/raw.array.json',
 'meta/root/zlib.array.json']

vs. (before this PR)

['blosc/lz4.array.json',
 'gzip.array.json',
 'raw.array.json',
 'zlib.array.json']

Similarly, meta/root/ would now show up in a similar call to list_dir:
ListDirResult(contents=['meta/root/gzip.array.json', 'meta/root/zlib.array.json', 'meta/root/raw.array.json'], prefixes=['meta/root/blosc'])

[grlee77]

Assuming this is the desired behavior, I will need to update the doctests accordingly

[grlee77]

now the CI just has the previously known failure raised in #37