Add methods to delete artifacts

[robherley]

Having official client methods to delete artifacts will make some use cases much simpler.
For instance, this could allow the ability to overwrite an artifact eventually in actions/upload-artifact.

e.g.

- uses: actions/upload-artifact@v4
  with:
    name: my-artifact
    path: path/to/artifact/
    overwrite: true # something like this

Where overwrite would delete an artifact first, allowing for the upload of another.

I'll update the documentation in a separate PR.

Closes: #380
Partially supports: actions/upload-artifact#471

[campersau]

Since there is no await for deleteArtifactPublic / deleteArtifactInternal is this warning actually printed when something goes wrong asynchronously? To me it looks this only catches errors which are thrown synchronously.