support for live templates - defines group "xquery" and "XQUERY_CODE" context

Initial support, only generic "XQUERY_CODE" context is defined. It may be useful to define additional
context, e.g context for templates defined only within function declaration
Issue #91 - Add prolog context. Add expression context. Add new templates (based on https://gist.github.com/dirkk/2fd14096038c1a6c6ea2).