Write Integration/E2E tests

Currently, `core` package is continuously changing, we need to have integration tests to make sure that new changes will not break the old features.

It's good to have [cypress](https://www.cypress.io/) tests on [example project](https://github.com/tuhuynh27/reactif/tree/master/example).

So after we have tests, we can perform CI tests check on every commits and pull requests.