Start BlockSparseArraysExt and testing

[mtfishman]

This PR creates a small KroneckerArraysBlockSparseArraysExt package extension for helping with compatibility between BlockSparseArrays.jl and KroneckerArrays.jl, specifically making it easier to work with block sparse arrays that have Kronecker arrays as blocks.

The goal is for both packages to be generic enough and featureful enough that working with block sparse arrays that have Kronecker arrays as blocks "just works", but of course that requires careful designing of each package to make that happen. A number of operations already work thanks to recent PRs I've made to both packages, I've started adding tests in this PR tracking which operations work and which ones are currently broken (marked with @test_broken).

Right now the package extension just defines a convenient constructor for making axes of block sparse arrays that have Kronecker arrays as blocks, namely it overloads the blockrange constructor for making a block unit range with Cartesian product blocks which enables writing code like the following:

using BlockArrays: Block
using BlockSparseArrays: blockrange, blocksparse
using KroneckerArrays: ⊗, ×

r = blockrange([2 × 2, 3 × 3])
d = Dict(
  Block(1, 1) => randn(2, 2) ⊗ randn(2, 2),
  Block(2, 2) => randn(3, 3) ⊗ randn(3, 3),
)
a = blocksparse(d, (r, r))

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.18%. Comparing base (a6336d1) to head (c21f3ac).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/KroneckerArrays.jl	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #16      +/-   ##
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+ Coverage   74.62%   79.18%   +4.55%     
==========================================
  Files           1        2       +1     
  Lines         532      538       +6     
==========================================
+ Hits          397      426      +29     
+ Misses        135      112      -23     

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