Make script load event asynchronous for internal scripts

[jdm]

According to https://html.spec.whatwg.org/multipage/scripting.html#execute-the-script-block:

If the script is from an external file, fire a simple event named load at the script element.
Otherwise, the script is internal; queue a task to fire a simple event named load at the script element.

This should be a Runnable object that contains a Trusted<HTMLScriptElement> value that gets dispatched to the script task of the element's containing window.