easy xml/json types

[Geokureli]

just an easy way to accept various args with OneOfTwo without having to convert them all to the final type for each method that uses the type as an arg

FlxXmlAsset: takes the following and converts to an Xml with getXml()

• a string path to an xml asset
• a xml string
• an Xml object

FlxJsonAsset<T>: takes the following and coverts to typedef T with getData().

• a string path to an json asset
• a json string
• a T instance