[API] Add endpoint to delete PRs

[jthvai]

Feature Description

TL;DR:

DELETE /repos/{owner}/{repo}/pulls/{index}

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This endpoint should be equivalent to the "Delete" button on a pull request's page (refer to screenshot below).

Screenshot: PR page with the Delete button highlighted

An equivalent endpoint doesn't exist in GitHub's API, but it would make sense to implement it like deleting an issue (where a DELETE request is sent to the same route as GETing an issue).

Screenshots

No response

[jthvai]

Closing as functionality is provided by DELETE /repos/{owner}/{repo}/issues/{index}; issues and PRs are the same internally.