Remove ndims type parameter from AbstractArrayStyle

[mtfishman]

This makes it easier to define subtype styles as singleton types, which is nicer for making aliases like const getindex_blocksparse = Implementation(getindex, BlockSparseStyle()).

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.04%. Comparing base (41e9512) to head (5c73a7d).

Files with missing lines	Patch %	Lines
src/style.jl	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #5      +/-   ##
==========================================
- Coverage   72.91%   69.04%   -3.87%     
==========================================
  Files           5        5              
  Lines          48       42       -6     
==========================================
- Hits           35       29       -6     
  Misses         13       13              

Flag	Coverage Δ
docs	31.57% <28.57%> (-0.98%)	⬇️

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