Allow use of the 'in' keyword in DRD variable names

The current grammar doesn't allow the use of the "in" keyword as part of variable names.

Improve the parser to allow the use of the "in" keywords for variables defined in the DRD as well as for variables in context entries.

The use of the "in" keyword will still be blocked as part of the "for" loop and "some"/"every" quantifiers.