adding WorldArgument

[DerEchtePilz]

This adds the WorldArgument suggested in #358
Feel free to merge it (or postpone it for a later version), I just wanted to implement this because it sounded simple enough for me xD

[DerEchtePilz]

Oops, forgot to add it to the Kotlin DSL

[JorelAli]

I appreciate the work put into this PR, but I'm closing this. After a long think about things,
the CommandAPI will only be accepting "custom argument implementations" that satisfy the following points:

• The custom argument is an implementation of a Vanilla Minecraft argument type that is specified in Argument types.
• The custom argument implementation heavily requires the use of Brigadier or NMS.

Hopefully, these two points above cover all bases, including new upcoming arguments that are added in future Minecraft updates, as well as complicated arguments such as the newly added CommandArgument.

The CommandAPI documentation already includes an example of a world argument here which makes use of the CustomArgument feature. This implementation using a CustomArgument does not require any additional libraries (e.g. Brigadier), nor does it require code which is hard/impractical to access (e.g. NMS). This makes it a perfect use case for end-user developers to implement themselves, rather than adding to the core CommandAPI.