feat: ModWeaponVars

[EladNLG]

The script part of the ModWeaponVars feature. Relies on Launcher#691

Usage

AddCallback_ApplyModWeaponVars( int priority, void functionref( entity ) callback ) adds a callback whenever we want to calculate weaponvars.

Then, you use ModWeaponVar_Set[type]( entity weapon, int weaponVar, [type] value )

We need a priority parameter because some operations override existing weapon variables (as in, setting a new value, and ignoring the current value), some add (as in, newValue = prevValue + modifier) and others multiply (as in, newValue = prevValue * modifier).

We generally want to go in the order of Overrides > Additives > Multipliers to avoid as many conflicts as possible (additives come before multipliers because multipliers are generally intended to multiply the entire result, and not have additions after them), and that's reflected in the WEAPON_VAR_PRIORITY_XXX constants.