Rewrite the demo app in TypeScript + React

The [current demo app][1] is [written in Elm][2], which is unpopular and has quite small ecosystem relative to JavaScript/TypeScript. In order to make it more maintainable and easier to contribute, it should be rewritten in more widely-used language and framework.

[1]: https://dahlia.github.io/seonbi/
[2]: https://github.com/dahlia/seonbi/tree/main/demo