This changes mixin handling for NeoForge.

TL;DR

• Use a mods.toml entry to register mixins to NeoForge now:

[[mixins]]
config = "modid.mixins.json"
[[mixins]]
config = "modid.more_mixins.json"

• Add extra config files to use with --fml.mixinConfig.
• The old manifest entry and --mixin.config options will still work, but they are discouraged. (Especially the option as it will cause issues if a dependency error occurs).

More detailed list of changes

• A mod can now add mods.toml entries to supply mixin configs.
  This approach will work both in dev and in prod, and means that we can get rid of the custom support in NG.
• Mod jars are not added to the GAME layer anymore if there is an error when computing the mod list.
  This means that mixins now get disabled if there is an error when computing the mod list (e.g. missing dependency).
• If mixins get disabled due to an error then the --mixin.config option will fail because it can't find the mixin config file. To fix this, this PR adds the new --fml.mixinConfig option that will only attempt to add the mixin configs after the loading mod list is computed.