Fix iterable return type

Currently, the PHPDoc states that a string should be returned, but that's not correct.

It can be anything that is echo-able.

That means any scalar, Stringable and even null.

src/Node/BlockNode.php

@@ -33,7 +33,7 @@ class BlockNode extends Node
         $compiler
             ->addDebugInfo($this)
             ->write("/**\n")
-            ->write(" * @return iterable<string>\n")
+            ->write(" * @return iterable<null|scalar|\Stringable>\n")
             ->write(" */\n")
             ->write(\sprintf("public function block_%s(array \$context, array \$blocks = []): iterable\n", $this->getAttribute('name')), "{\n")
             ->indent()