Add JS API

[srcrip]

The changes here are going to allow you to add a toast from the client side.

Note, that we are not actually constructing the element in javascript: it's merely a helper for sending a message to the live_toast LiveComponent to do the work server side. If someone has some need in the future to do it fully client side we could consider it.

[RodolfoSilva]

@srcrip I've described my expected behavior in #44.