Some Azure Policy JSON documents contain very long lines — ARM template
expressions with deeply nested if()/concat() calls can easily exceed
the default 1024-column lexer limit.
Add Parser::new_with_max_col() and corresponding parse_policy_rule_with_max_col()
/ parse_policy_definition_with_max_col() entry points so callers can raise
the limit when needed. Also bump ExprParser's own default to 65536 since
template expressions are routinely thousands of characters wide.