[MadNLPGPU] Refactoring - Part I

[frapac]

First part of MadNLPGPU's refactoring.

• rename all kernels as _{name_of_function}_kernel!
• move all code related to the KKT systems in a directory src/KKT
• move the linear solvers in another directory src/LinearSolvers
• format the code with JuliaFormatter.jl

[sshin23]

Agreed with the direction, but I'd first do a minor release

[sshin23]

LGTM!