Tokens: add T_FN to $parenthesisOpeners array

While as per [@gsherwood's comment](squizlabs#2523 (comment)) in 2523, arrow functions are not set up as scope openers, I think adding them to the `Tokens::$parenthesisOpeners` array would be justified.

I'm also wondering if the token should be added to the `Tokens::$weightings` array ?

src/Util/Tokens.php

@@ -362,6 +362,7 @@ final class Tokens
         T_ELSEIF     => T_ELSEIF,
         T_CATCH      => T_CATCH,
         T_DECLARE    => T_DECLARE,
+        T_FN         => T_FN,
     ];
 
     /**