Adds extensions for registration

[solonovamax]

Adds the following

• extensions for registering things to a Registry
• methods for some random builders (including ones that aren't necessarily for use in a registry)
• Introduces a dedicated silk-fabric module (included in the -all jar), as the fabric api was depended on in several places.

This is split off from #59.

[solonovamax]

@jakobkmar Given the addition of a silk-fabric module, should some of the other fabric-specific things be moved to it?
such as:

• any mixins (& anything that depends on them)
• all the fabric entrypoints
• anything that initializes things that paper might complain about

[jakobkmar]

@solonovamax I have to figure out the whole project structure (i.e. what to use, modules or source sets and how to order them - and how everything is published), I'll let you know once that is done.