Avoid array (but not argument) unpacking

Array unpacking requires PHP 7.4; array_merge is clearer in any case

lib/Docopt.php

@@ -152,12 +152,12 @@ class Docopt {
                 #     groups.append(child.children + children)
                 if ($child instanceof Either) {
                     foreach ($child->children as $c) {
-                        $groups[] = [$c, ...$children];
+                        $groups[] = array_merge([$c], $children);
                     }
                 } elseif ($child instanceof OneOrMore) {
-                    $groups[] = [...$child->children, ...$child->children, ...$children];
+                    $groups[] = array_merge($child->children, $child->children, $children);
                 } else {
-                    $groups[] = [...$child->children, ...$children];
+                    $groups[] = array_merge($child->children, $children);
                 }
             }
             # else: