Workflow: retest support

For workflows that are PR based (involving testing the merge head instead of the pr head), it'd be nice to be able to trigger a retest of the latest merge.

This could be because:

• the workflow points to a floating version of check spelling and thus retesting could give different results
• the base branch has changed
• the workflow is configured using external dictionaries that have changed

Conceptually this can be done using something like:

on:
  #...
  issue_comment:
    types: [created]
#...

jobs:
  spelling:
    if: ${{
        contains(github.event_name, 'pull_request') ||
        github.event_name == 'push' ||
        github.event_name == 'issue_comment' &&
        github.event.issue.pull_request &&
        contains(github.event.comment.body, '@check-spelling-bot retest')
      }}

To do this, a bit more glue would be necessary to teach the action side to treat this comment as a pull_request event...

I'm not entirely sure how much plumbing would be necessary from the action's side, there's already an event aliases option which might help a bit to map the comment to the pull_request handling...