Add full copy of zarr-python v2.18.7 under src/zarr/v2

[jni]

This proposes to include a complete copy of zarr v2.18.7 under the zarr.v2
namespace. The goal is for libraries using the v2 API to be able to migrate
gradually, and not have to be environment-incompatible with libraries that have
fully migrated to v3. Such libraries could then do:

import zarr

if not zarr.__version__.startswith('2.'):
    import zarr.v2 as zarr

It would also buy us more time to improve the migration guide.

For more discussion, see this Zulip thread.

Currently, I have not imported it in zarr.__init__.py, so one would have to
explicitly import it to discover it. I think that's probably the right thing.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.rst
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[jni]

Hmm looks like I have a lot of test fixing in my future. 😂

[jeromekelleher]

Thanks for doing this @jni - I think would personally very much appreciate this feature.