Improve sparse show internals

[mtfishman]

This will help with implementing better show methods for BlockSparseArrays.jl.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 65.53%. Comparing base (9a34fa2) to head (9f09958).
Report is 1 commits behind head on main.

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #14      +/-   ##
==========================================
- Coverage   65.77%   65.53%   -0.25%     
==========================================
  Files           6        6              
  Lines         263      264       +1     
==========================================
  Hits          173      173              
- Misses         90       91       +1     

Flag	Coverage Δ
docs	65.39% <66.66%> (-0.25%)	⬇️

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