Add GitHub actions for CI and publish

[doomspork]

This a work in progress.

[doomspork]

@btkostner you're a GitHub Action mastah, any thoughts on how we could handle tagging the commits + cutting a release?

[btkostner]

If I was doing this, I'd use semantic-release to analyze the git commits, set the semver version, write the CHANGELOG.md file, and cut the GitHub release.

The standard config includes an npm plugin which tries to push the release to npm. You should be able to just take that plugin out and it should all work.

I think this configuration should work for what you're trying to do. It's possible to clean it up a bit and remove the VERSION file and just set it with an optional env variable. Possibly also include the changelog file in the hexdocs. I think you'll need to use npm to install the plugin packages though.

https://github.com/semantic-release/semantic-release/blob/master/docs/usage/configuration.md#configuration-file

{
  "plugins": [
    "@semantic-release/commit-analyzer",
    "@semantic-release/release-notes-generator",
    "@semantic-release/changelog",
    ["@semantic-release/exec", {
      "prepareCmd": "echo \"${nextRelease.version}\" > ./VERSION",
      "publishCmd": "mix hex.publish --yes"
    }],
    "@semantic-release/git",
    "@semantic-release/github"
  ]
}

Bonus points to create a hex release plugin. Seems pretty simple based on this APM one:
https://github.com/semantic-release/apm/tree/master/lib
Alternatively, there is an exec plugin that will let you run your own scripts.
https://github.com/semantic-release/exec