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