Add NamedDimsArraysExt

[mtfishman]

Move NamedDimsArraysGradedArraysExt to GradedArraysNamedDimsArraysExt. That means breaking changes to GradedArrays won't require compat bumps for NamedDimsArrays, so I think it should make development a bit easier.

This change will require deleting NamedDimsArraysGradedArraysExt (ITensor/NamedDimsArrays.jl#136).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.65%. Comparing base (722a24a) to head (2f4ffa0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #84      +/-   ##
==========================================
+ Coverage   93.62%   93.65%   +0.02%     
==========================================
  Files          12       13       +1     
  Lines         926      930       +4     
==========================================
+ Hits          867      871       +4     
  Misses         59       59              

Flag	Coverage Δ
docs	0.00% <0.00%> (ø)

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