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

Applications/SystemMonitor/DevicesModel.cpp

@@ -127,10 +127,11 @@ void DevicesModel::update()
     if (!proc_devices->open(Core::IODevice::OpenMode::ReadOnly))
         ASSERT_NOT_REACHED();
 
-    auto json = JsonValue::from_string(proc_devices->read_all()).as_array();
+    auto json = JsonValue::from_string(proc_devices->read_all());
+    ASSERT(json.has_value());
 
     m_devices.clear();
-    json.for_each([this](auto& value) {
+    json.value().as_array().for_each([this](auto& value) {
         JsonObject device = value.as_object();
         DeviceInfo device_info;