feat: Support 'issue_comment' event as trigger for actions

[andekande]

This is intended to be used with lastComment validator, e.g.

version: 2
mergeable:
  - when: issue_comment.*
    validate:
      - do: lastComment
        must_include:
          regex: ^test$
    pass:
      - do: checks
        status: 'success'
      - do: assign
        assignees: ['@author']
      - do: comment
        payload:
          body: >
            @author assigned!
      # - do: close
      # - do: merge
      #   merge_method: 'squash'
    fail:
      - do: checks
        status: 'failure'

[github-actions]

🎉 This PR is included in version 2.19.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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