Provide Mixin as an optional project configuration

[Zidane]

Not looking for this project, outside of something optional, to be a mod development kit but since Mixin is so popular, we should provide a "path of least resistance" to getting Mixin up and manipulating classes.

Could replace MixinGradle usage in it's entirety for SpongeVanilla's usecase.

[Zidane]

If I recall, I think you had some ideas here @zml2008 .

[zml2008]

We don't do any remapping yet, so I don't think we'd get much benefit out of the AP, but what this submodule could do would be:

• Create the separate mixin/accessor source sets
• Configure the Mixin java agent on run configurations

Eventually, this could be extended to provide an obfuscation service for the Mixin AP to use.

[zml2008]

Another feature that would be nice: add a gradle task to automatically sort Mixin class names alphabetically in the src/*/resources/*.mixins.json files

[zml2008]

A further potentially useful tool:

Generate a diff from vanilla decompile to the classes produced by -Dmixin.debug.export=true

This would:

• execute some run task with the export enabled
• run the decompiler on those files with appropriate classpath
• diff to the existing vanilla sources