AK: JsonParser improvements

- Parsing invalid JSON no longer asserts
    Instead of asserting when coming across malformed JSON,
    JsonParser::parse now returns an Optional<JsonValue>.
- Disallow trailing commas in JSON objects and arrays
- No longer parse 'undefined', as that is a purely JS thing
- No longer allow non-whitespace after anything consumed by the initial
  parse() call. Examples of things that were valid and no longer are:
    - undefineddfz
    - {"foo": 1}abcd
    - [1,2,3]4
- JsonObject.for_each_member now iterates in original insertion order

DevTools/ProfileViewer/Profile.cpp

@@ -172,12 +172,13 @@ OwnPtr<Profile> Profile::load_from_perfcore_file(const StringView& path)
     }
 
     auto json = JsonValue::from_string(file->read_all());
-    if (!json.is_object()) {
+    ASSERT(json.has_value());
+    if (!json.value().is_object()) {
         fprintf(stderr, "Invalid perfcore format (not a JSON object)\n");
         return nullptr;
     }
 
-    auto& object = json.as_object();
+    auto& object = json.value().as_object();
     auto executable_path = object.get("executable").to_string();
 
     MappedFile elf_file(executable_path);