[feat] Add .once modifier

[justisr]

Sorry for the multiple issues in one go, but I figured this should go in its own one.

This seems very relevant to the caching system you're currently working on: #65

There are times when you only want to request a page's contents once, rather than every time the user clicks the button. For instance, your dialog/modal example https://alpine-ajax.js.org/examples/dialog/ every time a user clicks "Finn" another Ajax request is sent:

ab87306db0e108bec2e8a5c77e83a157.mp4

Ideally, I'd want to be able to keep the already downloaded modal and simply re-open it if the user clicks it again. This is particularly useful in cases where what's been downloaded is a form, and you want the user to be able to re-open it and continue filling in whatever they were working on, rather than downloading a new copy and erasing the previous contents.

Figured now would be a good time to suggest this since you're already working on a caching system.
The .once suggestion is purely based on Alpine's existing .once modifier for event listeners btw.