ZScript named arguments

ZScript supports the ability to add in named arguments. These arguments are useful for leaving other arguments blank in functions.

In order to use them, several conditions apply.

1. Required parameters cannot be skipped and must be filled out. Their names can't be used.
2. All parameters must be in the order as defined by the functions.
3. The name of the parameter must be used followed immediately with a colon (:)

// Skips xofs, yofs, yvel, and angle parameters. These are also in chronological order.
A_SpawnItemEx("Rocket", zofs: 32, xvel: cos(pitch) * 20, zvel: -sin(pitch) * 20, flags: SXF_NOCHECKPOSITION|SXF_TRANSFERPITCH|SXF_SETTARGET);