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