LibJS: Make RegExp.prototype.toString() spec-compliant

It should use the 'source' and 'flags' properties of the object, and
therefore work with non-RegExp objects as well.

Libraries/LibJS/Runtime/RegExpPrototype.cpp

@@ -248,10 +248,25 @@ JS_DEFINE_NATIVE_FUNCTION(RegExpPrototype::test)
 
 JS_DEFINE_NATIVE_FUNCTION(RegExpPrototype::to_string)
 {
-    auto* regexp_object = regexp_object_from(vm, global_object);
-    if (!regexp_object)
+    auto this_object = this_object_from(vm, global_object);
+    if (!this_object)
         return {};
-    return js_string(vm, String::formatted("/{}/{}", regexp_object->pattern(), regexp_object->flags()));
+
+    auto source_attr = this_object->get(vm.names.source).value_or(js_undefined());
+    if (vm.exception())
+        return {};
+    auto pattern = source_attr.to_string(global_object);
+    if (vm.exception())
+        return {};
+
+    auto flags_attr = this_object->get(vm.names.flags).value_or(js_undefined());
+    if (vm.exception())
+        return {};
+    auto flags = flags_attr.to_string(global_object);
+    if (vm.exception())
+        return {};
+
+    return js_string(vm, String::formatted("/{}/{}", pattern, flags));
 }
 
 }

Libraries/LibJS/Tests/builtins/RegExp/RegExp.prototype.toString.js

@@ -2,4 +2,5 @@ test("basic functionality", () => {
     expect(RegExp.prototype.toString).toHaveLength(0);
 
     expect(/test/g.toString()).toBe("/test/g");
+    expect(RegExp.prototype.toString.call({ source: "test", flags: "g" })).toBe("/test/g");
 });