@stephendolan, I have looked quickly at your code and for me, there isn’t much to say, it looks good, but I am no expert too and I agree with you that doing such package is a great way to learn.
For the experiment I did, I have used microbundle instead of Webpack as it was easier to use.
For Flatpickr, I have done this experimental package to see if it could bring some benefits to have a wrapper. In this case, I think it doesn’t add a lot of magic but in some case can make the integration in Rails lots easier.
The example of tab is a nice one as it is very common to have tabs and not all CSS framework provides the JS for tabs. So this ends up being quite reusable and handy (but you need to be aware that it exists).
This brings to one of your other discussion about some kind of aggregation site. Maybe a list of Stimulus resource would be nice, in the flavor of awesome lists but it could be branded modest list 