Test LinearSolve in vorticity formulation

[lukem12345]

When I wrote up the vorticity formulation Navier-Stokes example, I solved the Poisson problem using the default backslash operator for brevity.

However, for autodiff-related purposes, we should check that LinearSolve.jl can also be used.

Further, if improvements are made in either runtime or accuracy, the example should be updated to use it.