export plan_fwht functionality

[stevengj]

For people that care about performance (or multithreading, see https://github.com/JuliaMath/Hadamard.jl/issues/30#issuecomment-2797696797) when using this package, it would be good to export a function to create the plans separately, and then allow you to call mul!, similar to FFTW.jl.