Fix crash on performWorldGenSpawning

[blnchdev]

at net.minecraft.server.level.ServerLevel.handler$chc000$cupboard$OnaddEntity(ServerLevel.java:1656) ~[server-1.20.1-20230612.114412-srg.jar%23543!/:?]
at net.minecraft.server.level.ServerLevel.m_8872_(ServerLevel.java) ~[server-1.20.1-20230612.114412-srg.jar%23543!/:?]
at net.minecraft.server.level.ServerLevel.m_7967_(ServerLevel.java:815) ~[server-1.20.1-20230612.114412-srg.jar%23543!/:?]
at untamedwilds.entity.ComplexMob.breed(ComplexMob.java:239) ~[untamedwilds-1.20.1-4.0.4.jar%23529!/:4.0.4]
at untamedwilds.entity.relict.EntitySpitter.updateAttributes(EntitySpitter.java:353) ~[untamedwilds-1.20.1-4.0.4.jar%23529!/:4.0.4]
at untamedwilds.entity.ComplexMob.m_6518_(ComplexMob.java:422) ~[untamedwilds-1.20.1-4.0.4.jar%23529!/:4.0.4]
at net.minecraftforge.event.ForgeEventFactory.handler$dgh000$blueprint$onFinalizeSpawn(ForgeEventFactory.java:1569) ~[forge-1.20.1-47.4.10-universal.jar%23548!/:?]
at net.minecraftforge.event.ForgeEventFactory.onFinalizeSpawn(ForgeEventFactory.java:285) ~[forge-1.20.1-47.4.10-universal.jar%23548!/:?]
at untamedwilds.world.FaunaSpawn.performWorldGenSpawning(FaunaSpawn.java:151) ~[untamedwilds-1.20.1-4.0.4.jar%23529!/:4.0.4]

performWorldGenSpawning() -> onFinalizeSpawn() -> EntitySpitter.updateAttributes() -> ComplexMob.breed()

WorldGen spawning causes some ComplexMob instances to breed (maybe due to wantsToBreed always returning true?), which is illegal when called from a worldGen thread

So we just return early on breed() if the caller isn't the server main thread, this fixes the crash but I'm unsure if any features are affected