WebComponent build does not work

The ideal API we want for this library is demonstrated in https://github.com/learn-awesome/flashcard/blob/master/webcomp.html

However, this currently does not work. I couldn't figure how to make slots work well. Even the `fc-flash-card` component with `<fc-question>` and `<fc-answer>` nested children elements does not work.

This needs to be fixed for easier usage of this library.