Command target selectors

[dktapps]

Description

There are 3 tiers of this task, ranging from easy to hard:

• Implement basic (without arguments) @p (nearest player), @r (random player) and @s (self). These should be fairly easy as they each only support 1 target.
• Implement basic (without arguments) @a (all players) and @e (all entities). Many commands would have to be refactored to support multiple targets for these.
• Implement selector arguments. There are a ton of them I won't document here, but this would be a whole exercise of parsing in itself.

Open questions

I'm not sure what the default behaviour should be where multi-world is concerned. Should selectors target all worlds? Only the default world unless world=somethingElse is provided? Please discuss.

Justification

This is a needed feature that's gone ignored for a long time.

Alternative methods