2.0.0-alpha.3 for 1.21

Rewrote the spell execution engine to allow spells to seamlessly execute over longer spans of time. (@StellarWitch7, @ArkoSammy12, and @enjarai)
- Spells will now continue running in the background for as long as is needed for them to complete.
- Running spells can be killed using new buttons in the inventory.
- Spells can run indefinitely by recursing infinitely if needed.
Added tricks to display vertical bars with spell-provided values on the user's screen. (@enjarai)
Added a pattern for error handling within spells. (@StellarWitch7)
Improved spell editing slightly. (@enjarai)