Move close-pr workflow from react-native-bot to actions/github-script@v6 (#35500)

Summary:
This change adds a new action and workflow that will run whenever a commit is pushed that closes a PR via its message. When this happens, the corresponding PR will be labeled with "Merged" and a comment will be added with the name of the committer and the sha of the commit that resolved the PR.
This is not new functionality - it merely replaces a portion of the react-native-bot's functionality with a GitHub action.

## Changelog

[Internal] [Changed] - Moved close PR actions to a GitHub action

Pull Request resolved: https://github.com/facebook/react-native/pull/35500

Test Plan:
Tested the golden path here ("Merged" label and comment added): https://github.com/SlyCaptainFlint/react-native/pull/27
Tested the case where a PR already labeled with Merged was processed and no new comment was left on it (simulates the case where react-native-bot, which is still going to be running, got to the PR and processed it before the new GitHub action did): https://github.com/SlyCaptainFlint/react-native/pull/26

Reviewed By: cipolleschi

Differential Revision: D41567047

Pulled By: SlyCaptainFlint

fbshipit-source-id: 23745a17950b75c1a8f25f0b840c2172332c4cfa