Minor tidying of parser_nodes script
Summary:
Various minor improvements to parser_nodes.php, including:
- Use libphutil functions instead of vanilla PHP.
- phtize strings.
- Use an array instead of a heredoc for representing the nodes.
Test Plan: Ran the script and compared ouptut.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: epriestley
Differential Revision: https://secure.phabricator.com/D13994