Add a repository_dispatch hook for whatwg/sg updates

[foolip]

This is half of the solution, in addition the sg repository will need to
trigger the event:
https://developer.github.com/v3/repos/#create-a-repository-dispatch-event

Part of whatwg/sg#110.

[annevk]

Per https://github.community/t/triggering-by-other-repository/16163 it seems like this is the way to go, even if it's advertised for usage outside of GitHub.

I couldn't find documentation on types, but I'll trust you that it's correct.

[foolip]

I'm not 100% confident in types, but I based it on https://help.github.com/en/actions/reference/events-that-trigger-workflows#external-events-repository_dispatch and when this has landed I can experiment easily. (Don't feel like creating two other repos for testing.)