Fix message and broadcast with or list, redesign or ExpressionList getting

[TPGamesNL]

Description

Fixed EffMessage and EffBroadcast to handle 'or' expression lists correctly.
Also changed how a few of ExpressionList's methods work (getSingle, getArray and iterator), how the behaviour changed is explained in example below. The rest of the diff is formatting.

command /test:
	trigger:
		broadcast "a", "b" or "c"
		send "A", "B" or "C"
		
		set {_x} to "X"
		set {_y} to {_x} or {_none}
		broadcast "y: %{_y}

Before PR:

After PR:

ExpressionList would always try to pick a non-null value instead of picking a random value from the list.
I personally think the new behaviour from this PR makes more sense, but I'd like feedback on it (which is why I added 'up for debate' label)

---

Target Minecraft Versions: any
Requirements: none
Related Issues: #4543

[TheLimeGlass]

This doesn't mess up;

on inventory click:
    if index of clicked slot is 1 or 2:
        cancel event

by being randomly selecting one or the other? Making sure we're not introducing a new bug.

Also I think we should add tests for this, to ensure this doesn't break if it's implemented.

[TPGamesNL]

It doesn't break usage of ExpressionList in conditions, because conditions should use Expression#check, which is unaffected from this change.
The only effective change in ExpressionList is that {_x} or {_none} will no longer pick {_x} 100% all the time, as it didn't make sense to me, so I don't think this will break anything beside exactly the behaviour above.

[TheLimeGlass]

Ok, we should definitely add tests to maintain this standard. I can make some when I can if you don't before me.