Combine FusionStyles

[mtfishman]

This PR enables combining FusionStyles through the interface FusionStyle(::FusionStyle, ::FusionStyle), inspired by the interface for combining BroadcastStyles: https://docs.julialang.org/en/v1/manual/interfaces/#writing-binary-broadcasting-rules. Additionally, this PR combines the fusion styles of the inputs of matricized tensor contraction to determine a common fusion style (which should ensure the bases are compatible, assuming the matricization is defined properly).

Also add ITensorNetworksNext.jl as a downstream test.

This addresses an issue noticed in ITensor/ITensorNetworksNext.jl#27.

In the future, we should add a higher level interface for combining fusion styles based on the rules defined in this PR, see https://github.com/JuliaLang/julia/blob/98519108869cae69fa0622acdb3f1d24e174996d/base/broadcast.jl#L413-L483.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.39%. Comparing base (28e60c0) to head (a1d688f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #107      +/-   ##
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- Coverage   84.48%   84.39%   -0.09%     
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  Files          17       17              
  Lines         696      692       -4     
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- Hits          588      584       -4     
  Misses        108      108              

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