How to edit the CoreMod.java class so that it persists after building

In 2024.1, a new user code blocks will be added that will allow you to do this better:

Instead of locking base mod files, one can now add code within those custom user blocks and the code between those placeholder sections in the code will persist across builds, and if another generator specifies the same user code blocks, even across Minecraft versions, and even across modding APIs when possible.