Implement FormData event

[dfreedm]

The FormData event allows Custom Elements, and other elements, to add new data to a Form element before submission.

More info: https://www.chromestatus.com/feature/5662230242656256

[YonatanKra]

Note this is not supported in Safari

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[fernandopasik]

would it be possible to avoid this one from going stale?

[bicknellr]

I think these new comments should prevent the bot from closing this issue. FWIW, we're planning on coming back to this soon after we finish everything off for the next stable release of Lit.