Fixes async help unregister async

[TheLimeGlass]

Problem

Reloading scripts asynchronously that contain commands that have already registered their commands.

I use config.sk setting script loader thread size: 1 so this could be the culprit.

Caused by: java.util.ConcurrentModificationException
    at java.base/java.util.TreeMap$PrivateEntryIterator.nextEntry(TreeMap.java:1521)
    at java.base/java.util.TreeMap$ValueIterator.next(TreeMap.java:1566)
    at java.base/java.util.AbstractCollection.removeAll(AbstractCollection.java:370)
    at Skript.jar//ch.njol.skript.command.ScriptCommand.unregisterHelp(ScriptCommand.java:462)
    at Skript.jar//ch.njol.skript.command.Commands.unregisterCommand(Commands.java:267)
    at Skript.jar//ch.njol.skript.structures.StructCommand.unload(StructCommand.java:313)
    at Skript.jar//ch.njol.skript.ScriptLoader.unloadScripts(ScriptLoader.java:850)
    at Skript.jar//ch.njol.skript.ScriptLoader.reloadScripts(ScriptLoader.java:909)
    at Skript.jar//ch.njol.skript.ScriptLoader.reloadScript(ScriptLoader.java:898)

Skript 2.11.2
Paper 1.21.5

Solution

Unregister help commands on the main thread. Bukkit's help system isn't thread-safe.

[sovdeeth]

I think this is fine but I'd like pickle to review too