[Feature] Make CommandEvent cancellable

[PermissionError]

Issue

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Description

Recently I've been wondering why the CommandEvent is not cancellable. This could have a number of use-cases, primarily in the CommandListener#onCommand method, where a developer could do pre-command checks, and cancel the command execution if needed.

I mainly opened this issue to ask for some advice first, because in my mind this should be a reasonably simple modification to JDA-Utilities that I should be able to contribute, but if there are any problems or difficulties that I have overlooked, please tell me so.

[natanbc]

And how would a cancellable event work? You can just ignore it like a regular JDA event

[PermissionError]

@natanbc In the CommandClientImpl, it calls CommandClient#onCommand before actually executing thhe command. So, if we could do something like CommandEvent#setCancelled in the onCommand method, the CommandClient could then getCancelled, and if true, do not execute the command..

[jagrosh]

You should not be relying on the order events happen; do your checks via a category, and/or utilize exception handling to prevent commands from running