Implement "safe-casting"

[DerEchtePilz]

This PR is supposed to implement certain methods as suggested by Hawk here and by #535.

Currently, there is a bit more testing I want to do, additionally I am not sure if this would actually be something we want.
If so, there is a bit more work required:

• Global changelog entry
• Update CommandArguments, Documentation changelog

[willkroboth]

Just a note that some arguments use a raw type as their T generic parameter. For example, MapArgument<K, V> extends Argument<LinkedHashMap> rather than Argument<LinkedHashMap<K, V>>

https://github.com/JorelAli/CommandAPI/blob/a7d2717450c017002647fe611a3d39deda6874d2/commandapi-platforms/commandapi-bukkit/commandapi-bukkit-core/src/main/java/dev/jorel/commandapi/arguments/MapArgument.java#L26

If MapArgument did extend Argument<LinkedHashMap<K, V>>, then the getPrimitiveType method would have to return a Class<LinkedHashMap<K, V>> object, which Java doesn't like very much: https://stackoverflow.com/questions/1079279/class-object-of-generic-class-java.

That just means this code has a little warning:

MapArgument<String, Integer> argument = new MapArgumentBuilder<String, Integer>("map")
	.withKeyMapper(s -> s)
	.withValueMapper(s -> Integer.parseInt(s))
	.withoutKeyList()
	.withoutValueList()
	.build();

new CommandAPICommand("command")
	.withArguments(argument)
	.executes(info -> {
		// Type safety: The expression of type LinkedHashMap needs unchecked conversion to conform to Map<String,Integer>
		Map<String, Integer> map = info.args().getByArgument(argument);
		info.sender().sendMessage(map.toString());
	})
	.register();

This note might also be relevant for #545. It's not a big problem, but the two systems don't currently work perfectly for these arguments:

• AxisArgument
• BlockPredicateArgument
• EntitySelectorArgument.ManyEntities
• EntitySelectorArgument.ManyPlayers
• ItemStackPredicateArgument
• ListArgument and ListTextArgument via ListArgumentCommon
• MapArgument
• ParticleArgument
• ScoreHolderArgument.Multiple

[DerEchtePilz]

True, but if I am not mistaken, this is the best we can do here.
Since it additionally doesn't work for the CustomArgument anyway, I guess in terms of safety we can ignore the generic types.

[JorelAli]

A couple of minor documentation changes and code linting tweaks (avoid the use of variables which declared non-final and are only ever used once). A critical change required for PRIMITIVE_TO_WRAPPER