Add space for named ranges and accept indices in NamedTensor constructors

[mtfishman]

Summary

Adds space(i::NamedUnitRange), which returns the underlying range of a named range with the name dropped (equal to unnamed for a NamedUnitRange).

The NamedTensor/ITensor constructor now accepts indices (NamedUnitRanges such as Index) as the dimension names, not only plain names. It uses each index's name and takes the space from the array, so ITensor(array, (i, j)) works alongside the existing array[i, j] and ITensor(array, name.((i, j))) forms. A single bare index is still rejected, since it is ambiguous as the names collection.

Also drops the Compat dependency, declaring the package's public (non-exported) names with the native public keyword (guarded on VERSION so the package still loads on Julia 1.10).

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.09%. Comparing base (1066a99) to head (43a2f68).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/namedunitrange.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #218      +/-   ##
==========================================
- Coverage   76.10%   76.09%   -0.02%     
==========================================
  Files          30       30              
  Lines        1691     1694       +3     
==========================================
+ Hits         1287     1289       +2     
- Misses        404      405       +1     

Flag	Coverage Δ
docs	25.90% <42.85%> (-0.16%)	⬇️

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