fix: add parens around intersection type in union (DNF types)

[jorgsowa]

Fixes #2450

[elnebuloso]

Thanks for this fix, @jorgsowa!

I tested this branch on top of main (269374b) with

function f((A&B)|null $x): (A&B)|null { return $x; }

and the output keeps the parentheses: function f((A&B)|null $x): (A&B)|null. It passes php -l on PHP 8.5.11 and
keeps every token of the input. On main the parentheses are dropped, so the result no longer parses.

Would you be up for taking it out of draft? It would close #2450.