feat: add asyncDirective

[Accudio]

Adds a new function asyncDirective that allows for loading custom Alpine directives via a dynamic import. This will wait until Alpine executes a directive before downloading it.

This is very simple and just is a convenience wrapper for handling it yourself with dynamic imports, as Alpine.js supports async directives natively.

I'm not going to add conditional loading with a different API surface without significant demand for it due to amount of code for a relatively specific use-case.

Usage:

Alpine.asyncDirective('gsap', () => import('./src/gsap.js'))

I've just added the function version at the moment and it's the most practical, I'm happy to add URL and alias if it would be useful to people.

This work is primarily developed by @mickaelchanrion, from PR #56. Thank you very much to them for the work!

[Accudio]

Hi @mickaelchanrion, could you please give this a try and see if it meets your needs?

This is a rework of your PR #55 that relies more on Alpine's native async directive handling rather than extending parseName etc where it isn't required — with the goal of making it a bit simpler.

If this looks good I'll release this as version 2.1.0 asap.

[Accudio]

Test failure is due to module-crossorigin test which is not caused by this PR, and I'll look into separately. Happy to merge with that test failing.