[6.x] Backport ATs using Transform Action

[Jonathing]

Name entails. Part of FG7's new structure is the usage of Gradle's artifact transformers to apply AccessTransformers. This allows consumers to use mapped names from the Minecraft jar in their AT configs. I plan on backporting this to FG6, gated by a flag.

minecraft {
    accessTransformer = project.file('src/main/resources/META-INF/accesstransformer.cfg')
}

fg {
    newAccessTransformer = true
}

This would fix MinecraftForge/MinecraftForge#9954 for UserDev.