GitHub Actions CI

Workflows

On PR:
- build_and_test.yaml: builds and tests all combinations of OS & build settings
- update_docs.yaml: builds updated docs
On release tag:
- draft_github_release.yaml: creates a draft release on GitHub, triggers common build_and_test workflow
- build_and_test.yaml builds and tests all combinations of OS & build settings, attaches official binaries to the GitHub draft release, triggers publish_github_release workflow
- publish_github_release.yaml: finalizes the draft and published the GitHub release
- docker_hub_release.yaml: builds a Docker image to match the final GitHub release and pushes it to Docker Hub
- npm_release.yaml: builds an NPM package to match the final GitHub release and pushes it to NPM
- update_docs.yaml: builds updated docs, pushes them to the gh-pages branch

DOCKERHUB_CI_USERNAME: The username of the Docker Hub CI account
DOCKERHUB_CI_TOKEN: An access token for Docker Hub
- To generate, visit https://hub.docker.com/settings/security
NPM_CI_TOKEN: An "Automation"-type access token for NPM for the shaka-bot account
- To generate, visit https://www.npmjs.com/settings/shaka-bot/tokens and select the "Automation" type
SHAKA_BOT_TOKEN: A GitHub personal access token for the shaka-bot account, with workflow scope
- To generate, visit https://github.com/settings/tokens/new and select the workflow scope