feat: add MDARunner in experimental namespace

[tlambert03]

not sure if we'll ever want to merge this, This PR makes the MDARunner from pymmcore-plus available generically to non-micromanager users. (the runner only cares about MDAEvents ...). Of course, in order to be useful, someone would have to implement their own engine. But it's one option to add a "carrot" to useq-schema, and emphasize the separation of concerns...

I'm going to leave it in useq.experimental for now. maybe we can see how we like using it from the pymmcore-plus side

[codecov]

Codecov Report

Attention: Patch coverage is 95.40816% with 9 lines in your changes missing coverage. Please review.

Project coverage is 94.10%. Comparing base (a5819ef) to head (5e36e07).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/useq/experimental/_runner.py	93.61%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #197      +/-   ##
==========================================
+ Coverage   93.89%   94.10%   +0.20%     
==========================================
  Files          18       22       +4     
  Lines        1262     1458     +196     
==========================================
+ Hits         1185     1372     +187     
- Misses         77       86       +9     

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