Add closedby attribute

[scottaohara]

HTML recently landed the closedby attribute which can be used to light dismiss dialogs, or indicate that a dialog is not meant to be closed by user action (hitting esc key) until a specific task has been completed.

Filing this issue as a task to add this to the attributes table of HTML AAM, and also to get the group's opinions on whether there is some sort of mapping that might be useful to add here / potentially as an aria-* attribute - indicating that a dialog has these variant behaviors.

[cookiecrook]

https://html.spec.whatwg.org/multipage/interactive-elements.html#attr-dialog-closedby

[scottaohara]

Thank you for adding the spec link