Simplify SparseMatrix setindex! (#27026)
* Simplify SparseMatrix setindex!
Fix #27013, and add a whole slew of tests. `methods(setindex!, Tuple{SparseMatrixCSC, Vararg{Any}})` goes from a hodgepodge of 26 methods that were tough to reason about to just 6 methods: scalar, logical linear indexing, linear indexing with a vector, linear indexing with a matrix, generic nonscalar indexing, and a disambiguation method. This is much easier to reason about, and fixes a handful of bugs.
* : -> j
