Added Mozilla-compliant AST specification

Generic ECMAScript+JSX parsers and some other tools use extended Mozilla AST format as intermediate representation.

This PR describes those extensions for new spec consumers that want to work with AST.
@sebmarkbage I'd like to create it as separate document, too, but I wasn't sure how to create such other file that could be visible on site as separate page, so @vjeux recommended to just add this to main page for now.