Add Custom weapon implementation

[Ferrariic]

Goal: users would be able to click on the weapon drop down and click "Add New Weapon", this would pull up a "Build weapon" menu where they could select velocity, Moa, if vehicle or not, etc, and implement custom weapons into squad calc.

This would be beneficial for modders, as well as vanilla players experimenting with weapons in game.

This issue would remove #148, and #149 which look for mk19 implementation and instead replace it with a customizable solver.