[WIP] Rewrite based on NamedDimsArrays.jl/ITensorBase.jl

[mtfishman]

Rewrite based on NamedDimsArrays.jl and ITensorBase.jl. Relaunch of #1608 where implementations of the ITensor and Index types are located in ITensorBase.jl.

The idea here is that ITensors.jl will still contain things beyond the ITensor and Index types, like OpSum, operator definitions through the op interface, etc. to make upgrading ITensorMPS.jl and ITensorNetworks.jl easier.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (7faad33) to head (cd704ef).
Report is 18 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #1611       +/-   ##
==========================================
- Coverage   77.61%   0.00%   -77.62%     
==========================================
  Files          69      15       -54     
  Lines        5222    1266     -3956     
==========================================
- Hits         4053       0     -4053     
- Misses       1169    1266       +97     

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