Creates an executor for respecting the user script loader threads configuration

[TheLimeGlass]

Problem

Being able to submit runnables on the script loader configuration pool through an Executor.

Solution

Use Java's Executor to be able to submit runnables based on the users config.sk setting. Helpful for addon developers and Skript.
Also added support in Skript's Task util class.

Useful for things that needs to be submitted like streams.

Stream<String> testStream = Stream.of("test1", "test2", "test3");

// Take advantage of parallel and/or multiple threads configuration setting in a single call.
ScriptLoader.getExecutor().execute(() -> {
	(isParallel() ? testStream.parallel() : testStream).forEach(System.out::println);
});

See Pickle's comment at #7962 (comment)

Since my other Task util pull request got closed for some reason, I re-added my javadoc comments.