add Commands API

[SpaceWalkerRS]

Adds a Commands module and an implementation for (1.12)-(1.12.2) and (17w45a)-(1.13.2), with the following features:

• Command registration events.
• Brigadier implementation in 1.12.
• (that's it for the time being, I'll update this post as the API develops)

[SpaceWalkerRS]

I tried my hand at an API for client-side commands for 1.13, feedback is welcome! I still need to figure out autocomplete for client-side commands but registering and running them works so far.

[SpaceWalkerRS]

I added a first draft for the Brigadier implementation for 1.12 and below. It's mostly a port of the implementation as it is in 1.13, though I intend for the arguments to match the 1.12 legacy commands as closely as possible. For example, the coordinate arguments behave slightly differently, and 'local coordinates' do not exist in 1.12 and below. Some arguments I did already fix are entity selectors and blocks, those should already match the 1.12 legacy commands.