Get rid of jsonpath_gram.h and jsonpath_scanner.h

Jsonpath grammar and scanner are both quite small.  It doesn't worth complexity
to compile them separately.  This commit makes grammar and scanner be compiled
at once.  Therefore, jsonpath_gram.h and jsonpath_gram.h are no longer needed.
This commit also does some reorganization of code in jsonpath_gram.y.

Discussion: https://postgr.es/m/d47b2023-3ecb-5f04-d253-d557547cf74f%402ndQuadrant.com