Describe the issue

Per Handle's contract, #getHandle returns the object which the Terra API instance delegates to. Due to our direct implementation on Fabric, all #getHandle calls simply return this. Implementing #getHandle individually is tedious, redundant, and can cause non-fatal mixin conflicts if it's implemented in classes mixed into multiple times.

We should use Mixin's multi-target feature to implement #getHandle in one mixin, applying it to all relevant Minecraft classes simultaneously.