Support argument passing and return to do_file/string

[Trey2k]

The lack of support for this was just an oversight. do_file and do_string will now take an optional second argument that should be an array of arguments. And now return a Variant instead of a LuaError.

Example

func _ready():
	var lua: LuaAPI = LuaAPI.new()
	lua.bind_libraries(["base", "table", "string"])

	var ret = lua.do_string("""
	local a, b = ...
	print(a, b)
	return a + b
	""", [4, 6])
	if ret is LuaError:
		print("ERROR %d: %s" % [ret.type, ret.message])
		return
	
	print(ret)