Summary

Add workflow to do the following tasks:

• Fetch data from https://api.wordpress.org/core/stable-check/1.0/ and save it to wp-versions.json file.
• Create pull request against the artifacts branch in case no active pull requests are present.
• Push changes to the existing PR in case the PR exists.

Partially addresses #51
Related: wp-cli/wp-cli#5935

TODO (before merging this PR in the main branch)

• Create a new orphan branch called artifacts.
• Commit empty wp-versions.json file and push to the artifacts branch.
• Add required branch protection rules to avoid branch deletion by some automation or person.
• Enable Allow GitHub Actions to create and approve pull requests in Settings > Actions > General.