fix: defer to fsspec for memory url scheme

[d-v-b]

Only associate the "memory" scheme with ManagedMemoryStore when fsspec is not installed.

closes #3939

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/*.md
• Changes documented as a new file in changes/
• 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 93.23%. Comparing base (021662c) to head (adfb05d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3944   +/-   ##
=======================================
  Coverage   93.23%   93.23%           
=======================================
  Files          87       87           
  Lines       11695    11695           
=======================================
  Hits        10904    10904           
  Misses        791      791           

Files with missing lines	Coverage Δ
src/zarr/storage/_common.py	86.47% <100.00%> (ø)

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[d-v-b]

I added a test, when it passes I will merge this.

[maxrjones]

Thanks for fixing this, @d-v-b!