[Chapter 66] SpellUseSystem is never called

[PierreGuyot]

rustrogueliketutorial/book/src/chapter_66.md

Line 409 in 33872fe

This is *very* similar to the `ItemUseSystem`, but takes `WantsToCastSpell` as input. It then sends an `EffectType::SpellUse` to the effects system. We haven't written that yet - so let's do that. We'll start by adding it to the `EffectType` enumeration:

The text of the chapter describes how to write SpellUseSytem but forgets to add it to the run_systems function in main.rs.