feat: top-level function for running awaitables in zarr's event loop

[d-v-b]

following up on #3859 and the discussion in #3835, this PR adds a new top-level function run function that wraps the internal sync function. One notable difference between run and sync is that run raises a RuntimeError (like asyncio.run) when its invoked in a running loop.

While i would still prefer people go through higher-level apis for doing batch IO operations, we haven't gotten around to actually writing those APIs, and users need better perf today.

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)

[d-v-b]

@TomNicholas, @aladinor let me know how this looks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.50%. Comparing base (c0e2afa) to head (018a341).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3997   +/-   ##
=======================================
  Coverage   93.49%   93.50%           
=======================================
  Files          88       88           
  Lines       11873    11878    +5     
=======================================
+ Hits        11101    11106    +5     
  Misses        772      772           

Files with missing lines	Coverage Δ
src/zarr/__init__.py	100.00% <100.00%> (ø)
src/zarr/core/sync.py	94.54% <100.00%> (+0.20%)	⬆️

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