[WIP] Move ligand atom mapper from openfe to gufe

[atravitz]

related to OpenFreeEnergy/openfe#1617

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Checklist

• Added a news entry

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• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.76%. Comparing base (30e0c53) to head (6d75613).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #693   +/-   ##
=======================================
  Coverage   98.76%   98.76%           
=======================================
  Files          40       41    +1     
  Lines        2508     2519   +11     
=======================================
+ Hits         2477     2488   +11     
  Misses         31       31           

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[github-actions]

No API break detected ✅