Make event priority parsing more flexible

[TPGamesNL]

Description

I noticed that in skript-reflect Bukkit event listeners with priority weren't working properly due to the change in #3852.

Therefore, this change makes it so Skript attempts to parse the whole string as a normal event first, and after that it looks if there's a priority setting in it and uses the new string with priority for parsing and applies the priority afterwards.

Therefore, this change adds the method SkriptEvent#isEventPrioritySupport (by default overridden by SelfRegisteringSkriptEvent to return false), so addons can override this method if needed. I've also changed SkriptEvent#getEventPriority so that it returns the right value when called from SkriptEvent#init.

A quick test with a modified skript-reflect version shows everything still works:

on join:
	broadcast "normal"
on join with priority highest:
	broadcast "highest"
on join with priority lowest:
	broadcast "lowest"
on join with priority low:
	broadcast "low"

import:
	org.bukkit.event.player.PlayerJoinEvent

on PlayerJoinEvent:
	broadcast "ref normal"
on PlayerJoinEvent with priority highest:
	broadcast "ref highest"
on PlayerJoinEvent with priority lowest:
	broadcast "ref lowest"
on PlayerJoinEvent with priority low:
	broadcast "ref low"

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Target Minecraft Versions: any
Requirements: none
Related Issues: #3852