Enable customizing fusion of codomain and domain separately

[mtfishman]

This helps downstream packages like GradedArrays.jl with overloading the matricization interface.

[codecov]

Codecov Report

❌ Patch coverage is 84.37500% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.50%. Comparing base (03bb0a4) to head (d875538).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/matricize.jl	84.37%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #103      +/-   ##
==========================================
- Coverage   89.90%   89.50%   -0.40%     
==========================================
  Files          17       17              
  Lines         604      629      +25     
==========================================
+ Hits          543      563      +20     
- Misses         61       66       +5     

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

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