Adding thread lock to APICustomEMCMapper.customEMCforMod

[YukkuriC]

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Exact ProjectE version (do not say "latest", "latest on Curse", or similar):
1.20.1-PE1.0.1

Exact Forge version:
47.3.10

Link to crash log (please use a paste site, do not attach the .txt or paste the log inline):
https://paste.ubuntu.com/p/Dmm4Mjr969/

Steps to reproduce:

1. adding PE with some addons that add EMCs, mine were:
   • ProjectE Integration https://www.curseforge.com/minecraft/mc-mods/projecte-integration
   • Re-Avaritia https://www.curseforge.com/minecraft/mc-mods/re-avaritia
   • AppliedE https://www.curseforge.com/minecraft/mc-mods/appliede
2. enter game

What I expected to happen:
get to main menu

What happened instead:
occasionally, during FMLCommonSetupEvent, multiple mods access to APICustomEMCMapper.registerCustomEMC at the same time, then they load fail with java.util.ConcurrentModificationException