Finalise `_MeshIndexSet` work from #7149

[trexfeathers]

• _MeshIndexSet first pass #7149

This work is to be performed on the FEATURE_index_set feature branch.

Split off into a separate issue to allow better work breakdown and avoid perpetual WIP status. Finishing this should complete #6122

TODO list

These are ALL marked with # TODO comments in the code.

• Type hinting for _MeshXYMixin core attributes
  Ref
• Type hinting for connectivity/coord manager attributes
  Ref
• Backwards compatibility no-op for Mesh.from_coords()
  Ref
• Docstrings for _ManagerMembers
  Ref
• Update _MeshIndexSet docstring
  Ref
• Decide between private class or experimental class for _MeshIndexSet
  Ref
• Consider validating inputs for _MeshIndexSet __init__
  Ref
• Finish type hinting for _MeshIndexSet
  Ref
• Docstrings for _MeshIndexSet
  Ref
• Informative error when attempting to save `_MeshIndexSet
  Ref
• Make __getstate__ an abstract method in MeshMixin
  Ref
• Standardise "_MeshIndexSet_NotImplemented" as a string constant
  Ref
• Use match-case in _calculate_node_indices
  Ref
• Consider better validation in _calculate_node_indices
  Ref
• Only re-index the coord manager when there has actually been a change
  Ref
• Only re-index the connectivity manager when there has actually been a change
  Ref
• Re-order components.py so that the manager classes are defined before the mesh classes
  Ref
• Consider validating MeshCoord __getitem__ keys as 1-dimensional
  Ref
• Handle problems with single index requests in MeshCoord __getitem__
  Ref
• Consider checking MeshCoord.location against mesh_index_set.location in __getitem__
  Ref
• Fix MeshCoord test failures:
  • https://github.com/SciTools/iris/pull/7149/changes#diff-61625697de31a7fce2441c0214e4086666ac937761e6af4232a65a47a5bba1f9R53
  • https://github.com/SciTools/iris/pull/7149/changes#diff-61625697de31a7fce2441c0214e4086666ac937761e6af4232a65a47a5bba1f9R68
  • https://github.com/SciTools/iris/pull/7149/changes#diff-937635c5575e5a8a398c820da1d8de47bf352e4ea84f32daa861a80f1528af44R232
  • https://github.com/SciTools/iris/pull/7149/changes#diff-937635c5575e5a8a398c820da1d8de47bf352e4ea84f32daa861a80f1528af44R500
  • https://github.com/SciTools/iris/pull/7149/changes#diff-366beba0ee21583d7672c6f3c078709358f7d5776bfcd9f2dc177c641fae2b4aR277