feat: add WithWorldGuard and HandlerContext system parameters

[makspll]

This should allow things like:

fn my_system(mut with_guard: WIthWorldGuard<HandlerContext<LuaScriptingPlugin>>) {
	let (world_guard, handler_ctxt) = with_guard.get_mut();
	handler_ctxt.call("my_script" , ... // etc )
}

In theory it also lets you call scripts in ANY system, however the world guard will know about the other resources/ components being accessed, meaning that it will not be possible to access &mut from scripts if you access ANYTHING else (other than the handler context).

This is a good first step towards #244 , as it demonstrates how systems and WorldGuard should interact.

Also moves tests to assets to make them more discoverable as examples, and adds Default implementations for all the main resources, so they can be std::mem::replaced