Add commands.complete()

[JakobDev]

This function complete a Command like the Chat. e.g. commands.complete("time ") will return the table {"set","add","query"}.

Another Usage of this function is getting information from MC. e.g. commands.complete("playsound ") will return a list of all available sounds and commands.complete("setblock ~ ~ ~ ") will return a list of all blocks.

[SquidDev]

Whilst this'd be a really neat feature to have, I'm slightly worried about thread safety. The completion generator will need to run on the server thread, so we'll probably want complete and completeAsync variants.

However, this'd then make it unusable as a completion provider to read as it'd introduce a delay in typing. One could probably adjust the completion provider to rus in parallel to reduce the latency, but I'm not sure how much it's worth it.

[thatcraniumguy]

Can one of the admins verify this patch?