Add a method for mod dependent mixins

[Chocohead]

Related to #68

Just an idea to save people needing mixin plugins for mixins that depend on other mods in a way that doesn't need changes within Mixin itself. Not the prettiest passing FabricLoader in, but it was the best I could come up with (given it's an internal interface anyway). Did consider allowing full dependency ranges rather than just "is the mod present" but didn't want to over invest time in something that might not be what we want anyway.

[modmuss50]

I do think having version ranges is something that would be worth having. One use case would be for handling diffrent game versions.

[sfPlayer1]

I plan on having something like it, but only alongside the metadata version bump I'm working on (brings a bunch of other additions too)