Simplify spellcasting.

[viking-sudo-rm]

I've decided to simplify spellcasting a bit. Now, each item implements an onCast method, which takes a CastEvent. This allows information about cast position, etc. to be passed in from the context.

Note that sy (it/mouse) will always refer to the dynamic (i.e. runtime) mouse position, whereas the second person object now refers to either the object that the spell was cast on or a blank entity at the initial cast position.

One immediate consequence of this is that Open will be refactored to be intransitive. It will always use the second person object as the position for opening. This makes its behavior more intuitive. Just write wpi on a sling or target a papyrus with wpi on a door and it should work as intended.

Since using Open is the first step in the tutorial, it lets us start with a non-compositional spell, and then introduce a compositional spell in stage 2.