Proposal: whatwg/html#9373

Code based on https://github.com/keithamus/invoker-buttons-proposal

This adds a new closedBy property and closedby attribute to the HTMLDialogElement.

Keyword	Description
'none'	No automatic closing.
'closerequest'	Close watchers trigger close
'any'	Light dismiss and close watchers trigger close

The default is an 'auto' behaviour with no matching keyword. This maintains existing behaviour, 'closerequest' for modal dialogs, else 'none'.