Add Event Priority to Event Listeners

[graywolf336]

What? :boat:

• New method decorator which sets the priority of the event

Why? :thinking:

• So app developers can determine the priority of their listeners (mostly useful for private apps that must have a certain order)

Links :earth_americas:

#453

PS :eyes:

@SetEventPriority(EventPriority.HIGHEST)
public async executePreMessageSentModify(message: IMessage, builder: IMessageBuilder, read: IRead, http: IHttp, persistence: IPersistence): Promise<IMessage> {
}