Viper Alt-Fire not Swapping Pistols

[UndeadZeratul]

https://github.com/HDest-Community/ace-corp-extended/blob/main/zscript%2Faccensus%2Fweapons%2FViper%2Fviper.zs#L481-L484

Seems like an easy fix, the weapon help calls for using Alt-Fire to swap pistols but there isn't a state defined for it, and Firemode is simply laid on top of the actual SwapPistols state, so hopefully just adding AltFire to the list should do it.