feat: add pull_request.{demilestoned, milestoned} events

[JamieMagee]

Closes #223

Resolves #ISSUE_NUMBER

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• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Added the appropriate label for the given change

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• Bugfix: Type: Bug
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• Dependencies/code cleanup: Type: Maintenance

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