Variable Spawner Hardness

Ported to 1.21.10, performance improvements.

• Using a switch that returns constant values is faster than indexing on mutable array contents (the array itself was static final, but there's currently no public API way of telling the JVM that the array contents won't change - JDK internals have `@Stable` but can't use it in this mod)
• Grab level data once to reduce the number of untrusted final field reads (level#getDifficulty calls level#getLevelData)