feat: accept bare string for mda event channel

[tlambert03]

MDAEvent(channel="DAPI") == MDAEvent(channel={"config": "DAPI"})

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.13% 🎉

Comparison is base (74f2690) 98.01% compared to head (b5347a0) 98.15%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
+ Coverage   98.01%   98.15%   +0.13%     
==========================================
  Files          14       14              
  Lines         808      811       +3     
==========================================
+ Hits          792      796       +4     
+ Misses         16       15       -1     

Files Changed	Coverage Δ
src/useq/_mda_event.py	90.19% <100.00%> (+2.69%)	⬆️

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