Avoid doing-it-wrong notices with direct calls

[lucatume]

This PR fixes an issue where the wp_script_is, wp_style_is, wp_register_script and wp_register_style functions would be called too early, triggering a doint-it-wrong PHP notice.

Calling those functions before at least one of the init, wp_enqueue_scripts, admin_enqueue_scripts or login_enqueue_scripts actions has fire, will trigger a doing-it-wrong notice.

The change consists in running the same checks those functions would call to know if the registration would happen too early, if that is the case, the asset will be queued later.
I think the library API should abstract away the "is it too early?" concern, and just do it when it's right.

[borkweb]

This looks like a good change! I dug around a bit to ensure that the auto-registering happened and it looks like it does nicely. That being said, there's no tests for that (my bad)

I think it'd be a good idea to add some Acceptance tests that ensures assets still get registered and enqueued if registered earlier than init.

[lucatume]

I think it'd be a good idea to add some Acceptance tests that ensures assets still get registered and enqueued if registered earlier than init.

Done.