Add Mixed-LCP

[tmigot]

Following what has been done the LCP:

A(u,v) + a = 0
0 <= v _|_ B(u,v) + b >=0

[arnavk23]

@tmigot If you like Tangi, I would also like to work on this repo. If you want to revive this repo, do tell.

[tmigot]

If you really crave for work, making this one work https://github.com/tmigot/MPCC.jl would be better