Refs #2215

Possible options (#2215 (comment), summarized):

• create an Xml doc with all the possible constructs of the language and generate an xsd for it.
• provide a list of elements and attributes manually or hand-craft an xsd for output.
• generate it from the grammar when supported

I like better the third option, though we'll need a different generation method for Antlr & Javacc grammars, and for wrapper trees like Apex. #2211 is a prerequisite, hence the 7.0 milestone