LibJS: Tweak FunctionPrototype::to_string and constructors

The output of FunctionPrototype::to_string is now more in line
with the output in Firefox. The builtin constructors have been
extended to include their function name in the output.

Libraries/LibJS/Runtime/ErrorConstructor.cpp

@@ -31,6 +31,7 @@
 namespace JS {
 
 ErrorConstructor::ErrorConstructor()
+    : NativeFunction("Error")
 {
     put("prototype", interpreter().error_prototype());
     put("length", Value(1));