Fixes #6121

It was a bit tricky to fix because the formatter works like this:

• To format an expression, like 1 or 1 + 2, the formatter traverses that and outputs/fixes that (corrects whitespace, etc.), but it doesn't insert a newline
• Newline is inserted after an expression is formatter only if more expressions come, or if it's before an "end", etc.
• But a heredoc already ends with a newline (it's probably the only expression with this characteristic), so a second newline must not be placed in the above cases

That's why a newline is now inserted after writing a heredoc, and in some places I check that if a newline was already printed, don't print it again.

The issue also happened with other closing stuff, like }}, ], }, not only %}.