Automate the release process for gatsby-theme-carbon

[tay1orjones]

The release process for this package needs to be automated so that any push to the main branch will publish a new minor to npm and create a release on GitHub.

This is required so that we can support the workflow of

1. Carbon releases it's packages @carbon/react, etc.
2. gatsby-theme-carbon is updated to latest carbon deps and publish an automatic release
3. carbon-website can then pick up both the Carbon dependency updates and gatsby-theme-carbon updates into a single PR.

The existing tooling, release-it, supports a --ci option and should be fairly easy to automate.

[eng618]

Thanks, @tay1orjones, I'll definitely work on this and get it in place soon.

[eng618]

The PR still needs some adjustments, since it's not able to authenticate to GitHub and actually push/publish the release...

[eng618]

3.1.7 was auto published