Move SparseArrays to an extension

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (cb486f9) 86.97% compared to head (fac6b61) 86.97%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #163 +/- ## ======================================= Coverage 86.97% 86.97% ======================================= Files 10 11 +1 Lines 1190 1190 ======================================= Hits 1035 1035 Misses 155 155 ``` | [Files Changed](https://app.codecov.io/gh/JuliaLinearAlgebra/BlockBandedMatrices.jl/pull/163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra) | Coverage Δ | | |---|---|---| | [src/BandedBlockBandedMatrix.jl](https://app.codecov.io/gh/JuliaLinearAlgebra/BlockBandedMatrices.jl/pull/163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-c3JjL0JhbmRlZEJsb2NrQmFuZGVkTWF0cml4Lmps) | `86.82% <ø> (-0.77%)` | :arrow_down: | | [src/BlockBandedMatrices.jl](https://app.codecov.io/gh/JuliaLinearAlgebra/BlockBandedMatrices.jl/pull/163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-c3JjL0Jsb2NrQmFuZGVkTWF0cmljZXMuamw=) | `33.33% <ø> (ø)` | | | [ext/BlockBandedMatricesSparseArraysExt.jl](https://app.codecov.io/gh/JuliaLinearAlgebra/BlockBandedMatrices.jl/pull/163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-ZXh0L0Jsb2NrQmFuZGVkTWF0cmljZXNTcGFyc2VBcnJheXNFeHQuamw=) | `100.00% <100.00%> (ø)` | |

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