Improve heredoc syntax checkers

Parser validation finds and reports heredocs syntax issues Heredoc syntax checking is now performed during validation, unless the heredoc contains any interpolated expressions. This includes manifest and EPP template syntax checks. Heredocs with interpolation are checked during evaluation. Prior to this release, heredoc syntax checks were run only during evaluation. By default, heredocs are text unless otherwise specified with the end-of-text tag, such as @(EOT:json), @(EOT:app), or @(EOT:epp). To check syntax in heredocs, run `puppet parser validate`.