Revert changes to cat in AbstractArrayOps, introduce cat_along

[mtfishman]

This reverts a breaking change to AbstractArrayOps introduced in #13, and reverts the removal of the interface version of Base.cat, which broke BlockSparseArrays.jl.

It also introduces a new interface function cat_along(dims, as::AbstractArray...) which is like Base.cat(as::AbstractArray...; dims) but where the dimensions are passed as the first argument (so basically the private Base function Base._cat with a nicer name). We can discuss the name and interface later, mostly it is meant as an initial proposal.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 72.89%. Comparing base (51b1cf7) to head (b416bc8).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/abstractarrayinterface.jl	60.00%	2 Missing ⚠️
src/traits.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
- Coverage   73.18%   72.89%   -0.29%     
==========================================
  Files           9        9              
  Lines         317      321       +4     
==========================================
+ Hits          232      234       +2     
- Misses         85       87       +2     

Flag	Coverage Δ
docs	37.73% <0.00%> (-0.49%)	⬇️

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