Add configurable main jar task and jarJar integration

[MelanX]

This PR adds support for using JarJar in ModGradle easily, especially for publishing the artifacts.

You can use it like this:

mod.configure {
    artifacts {
        useJarJar()
    }
}

That way, the main jar task for publishing isn't the default jar task anymore but instead jarJar. If using useJarJar(tue), it also switches the classifier of the default task to thin and the jarJar task classifier to an empty string.