LibJS: Implement Array length setter

Author: https://github.com/linusg
Commit: 418092a71a
Pull-request: https://github.com/SerenityOS/serenity/pull/1929

Libraries/LibJS/Runtime/Array.h

@@ -30,6 +30,8 @@
 
 namespace JS {
 
+Array* array_from(Interpreter&);
+
 class Array final : public Object {
 public:
     static Array* create(GlobalObject&);
@@ -47,5 +49,4 @@ private:
     static void length_setter(Interpreter&, Value);
 };
 
-
 }