Add a freemarker variable for adding non reserved keywords to Parser.jj template

Currently Calcite parser allows customizing parser grammar for supporting new Sql constructs (ex. SHOW TABLES or CREATE TABLE ... AS QUERY) through freemarker template variables. There is a freemarker template variable to allow new keywords, but all these keywords are added as reserved keywords which are not allowed as identifiers unless escaped with ` (or configured casing character).

This JIRA is to add a freemarker template variable for adding non reserved keywords list.