(#15920) Make forward-sexp ignore comments in puppet-mode.el

Enables parse-sexp-ignore-comments.

Previously forward-sexp was broken by comments with unterminated
strings. E.g. with the following manifest:

foo {
  # won'
}

forward-sexp on the opening brace would get:

forward-sexp: Scan error: “Unbalanced parentheses”

The problem got introduced in commit
1c02749e8b54616043f728ed18ddec1dc4353a2d (Committed patch from #1160).