LibJS: Implement constructor/non-constructor function calls

This adds Function::construct() for constructor function calls via `new`
keyword. NativeFunction doesn't have constructor behaviour by default,
ScriptFunction simply calls call() in construct()

Libraries/LibJS/Runtime/Function.h

@@ -36,6 +36,7 @@ public:
     virtual ~Function();
 
     virtual Value call(Interpreter&) = 0;
+    virtual Value construct(Interpreter&) = 0;
 
 protected:
     Function();