Moves all CI to GitHub Actions

[vlj91]

• Adds the remaining steps to GitHub Actions: lint, build, and publish. There is a slight change in that we now always run the lint and build steps
• We also now perform the lint, build, and publish steps using the Node version in .nvmrc, rather than explicitly using Node 14 as we did in CircleCI
• Removes the CircleCI configuration

Once approved, I'll disable CircleCI prior to merging.