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/NativeFunction.h

@@ -37,6 +37,9 @@ public:
     virtual ~NativeFunction() override;
 
     virtual Value call(Interpreter&) override;
+    virtual Value construct(Interpreter&) override;
+
+    virtual bool has_constructor() const { return false; }
 
 protected:
     NativeFunction() {}