Add assignees when a draft PR is marked ready for review

[arminru]

Is your feature request related to a problem? Please describe.
Currently assignees are not added when a draft PR is marked ready for review, only when it is opened as ready right away.

Describe the solution you'd like
I'd expect assignees to be added once the draft PR is marked ready for review.

Additional context
I noticed that #105 introduced behavior intended to add assignees only once marked as ready, but apparently this does not work as expected. It added the following hook:

auto-assign/src/index.ts

Line 6 in 71d1a9c

app.on('pull_request.ready_for_review', handlePullRequest)

If I understand the (quite sparse) documentation correctly, this hook/event does not apply when a draft PR is marked as ready for review but only when a non-draft PR is created, so the handler will not be called a second time.