Cosmetic changes for jsonpath_gram.y and jsonpath_scan.l

Cosmetic changes for jsonpath_gram.y and jsonpath_scan.l

This commit include formatting improvements, renamings and comments.  Also,
it makes jsonpath_scan.l be more uniform with other our lexers.  Firstly,
states names are renamed to more short alternatives.  Secondly, <INITIAL>
prefix removed from the rules.  Corresponding rules are moved to the tail, so
they would anyway work only in initial state.

Author: Alexander Korotkov
Reviewed-by: John Naylor