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/ScriptFunction.cpp

@@ -55,4 +55,9 @@ Value ScriptFunction::call(Interpreter& interpreter)
     return interpreter.run(m_body, arguments, ScopeType::Function);
 }
 
+Value ScriptFunction::construct(Interpreter& interpreter)
+{
+    return call(interpreter);
+}
+
 }