Create global Alpine.$persist to enable persisting in Alpine.store

[MalcolmKnott]

Until now it has been impossible to persist values in an Alpine.store

This PR extracts the $persist magic to a globally accessible Alpine.$persist to enable use cases such as:

Alpine.store('darkMode', {
    on: Alpine.$persist(true)
})

The $persist magic is still registered, so this is a non-breaking change for exisiting $persist users.

We've included a test, as well as an index.html that makes it easy to play with the feature. Feel free to remove the index.html before merge. We also fixed a small bug that made rerunning the persist.spec.js in the same browser session not work.

@DanielCoulbourne says hi.

[DanielCoulbourne]

This right here is a grade A pull request! Only a FOOL would neglect this stunning contribution 💯

[SimoTod]

Great addition! if you want to reset the index.html you can do git checkout master -- index.html.
It would be good to update the docs as well, it's in package/docs/src/en/plugins/persist.md

[calebporzio]

Thanks @MalcolmKnott! Good call @SimoTod - I agree - just do some cleanup on the PR (index.html and any stray extra lines) - then add docs to the Markdown file and we'll be good to goooooo

[MalcolmKnott]

@calebporzio Should be ready for review now :)

[philwolstenholme]

Small typo here @MalcolmKnott:

Suggested change

	`Alpine.$persist` is exposed golobally so it can be used outside of `x-data` contexts. This is useful to persist data from other sources such as `Alpine.store`.
	`Alpine.$persist` is exposed globally so it can be used outside of `x-data` contexts. This is useful to persist data from other sources such as `Alpine.store`.

[calebporzio]

Thanks @MalcolmKnott! This will be much appreciated by the community! Great work!