Feature: enqueue script and stylesheet with notice

[JasonTheAdams]

Especially with custom notices, it's very likely that the custom notice will have it's own styles. It's also quite plausible that it will have custom JS as well. It's currently possible, of course, to enqueue these manually in the admin, but it would be a lot of work to get the scripts to only enqueue when the notice is set to display.

This PR introduces AdminNotice::enqueueScript() and AdminNotice::enqueueStylesheet methods. This makes it nice and easy to include a script and/or stylesheet as part of your notice!

AdminNotices::show('custom', function(AdminNotice $notice, NoticeElementProperties $elements) {
    return "<div class='fancy-custom-notice'>
         <marquee>It's <strong>Stellar</strong> at <strong'>StellarWP</strong></marquee>
         <span class='close' {$elements->customCloserAttributes()}>hide</span>
    </div>";
})
    ->custom()
    ->enqueueScript($pluginUrl . 'custom-notice.js')
    ->enqueueStyle($pluginUrl . 'custom-styles.css');

And, of course, if the notice is not going to display — either due to conditions or user dismissal — then the files will not be loaded. Finally, the script is deferred by default, to keep things performant.

[borkweb]

This looks great to me!