Add zstd to old V3 supported codecs

[rabernat]

This adds zstd to the list of codecs supported by the old V3 code path. This is essentially a backport to support existing data that was already written this way.

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/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.98%. Comparing base (bf89533) to head (07aab24).
Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1914   +/-   ##
=======================================
  Coverage   99.98%   99.98%           
=======================================
  Files          38       38           
  Lines       14695    14703    +8     
=======================================
+ Hits        14693    14701    +8     
  Misses          2        2           

Files	Coverage Δ
zarr/codecs.py	100.00% <100.00%> (ø)
zarr/meta.py	100.00% <100.00%> (ø)
zarr/tests/test_meta.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[rabernat]

Note for review: I made this change to get test coverage to 100%.

[jhamman]

Thanks @rabernat! We can make a patch release with this in it.