[ENHANCEMENT] Define SparseArraysBase.sparse to convert from dense to sparse, dropping zero values

[mtfishman]

See: https://sparsearrays.juliasparse.org/dev/#SparseArrays.sparse.

[mtfishman]

Relatedly, it would be nice to define SparseArraysBase.dense for generically converting back to a dense array. Obviously you could use Array for that most of the time, but dense could be more generic and preserve some property about the sparse array, such that it is on GPU.

In addition, we should define something like BlockSparseArrays.sparseblocks and BlockSparseArrays.denseblocks for preserving the block structure and sparsity but making the blocks themselves sparse or dense. That's obviously an issue for BlockSparseArrays.jl but would rely on this functionality.