Setting .inert property doesn't work if script is async

[robdodson]

If inert.js is loaded as <script src="inert.js" async> and I select an element and set its .inert=true property before the script has had a chance to load, that inert property will not get picked up by the initial pass in _onDocumentLoaded. I believe you must use the inert attribute, instead of the property, for initial setup, if you're loading the script async.

Would it be possible to inspect to see if an element has set its inert property when we patch the prototype? If not, we may want to indicate in the docs that this is a gotcha.

cc @alice @paullewis