[hackjs] skeleton for pretty printer of ast_php_simple

Summary: Avik may need that at some point

Test Plan:
$ make
$ ~/pfff/pfff_test -pp_php_simple ~/pfff/demos/foo.php
function foo(...) {
call;
}
function foo2(...) {
call;
}
call;
call;

Reviewers: bhosmer, avik, jeffmo

Reviewed By: bhosmer

Differential Revision: https://phabricator.fb.com/D996507