Support for ModLauncher

[ZeroMemes]

I'm not sure of the current state of ModLauncher in terms of its functional capabilities, however, this should be a long term goal.

LegacyLauncher should effectively be separated from the codebase to allow for a more modular system that is capable of supporting both LegacyLauncher and ModLauncher. (Mixin has already taken this route)

[babbaj]

not sure how you're supposed to do tweakers with the new mod launcher but i can probably just do whatever mixin does