Variable Spawner Hardness

Rewritten under a new multiloader architecture, full feature parity with the Forge version. Uses Mixin and custom code instead of Fabric API because they don't offer the things this mod needs (a config API and an equiv to Forge's PlayerEvent.BreakSpeed)

Also: Spawner break speed changes now only affect players on Fabric rather than all mobs. This was an issue specific to the Fabric version of the mod and this fix now brings it more in line with how the mod behaves on other loaders.