Create new release when merge requests are merged into the
`$STABLE_BRANCH` branch.

Create prerelease on push to `$TESTING_BRANCH`

* .gitlab-ci.yml: execute a new stage `release` for the new
  `semantic-release` include.

* .releaserc.cjs: use `.cjs` extension to avoid being treated as ES
  module file.
  Configure `stableBranch` for release and
  `betaBranch` for prerelease.
  Create `docs/CHANGELOG.md` for stable branch and publish new
  releases on Gitlab.
  Build npm package and publish the tarball with the release.
  Create semantic version update rules based on
  `docs/CONTRIBUTING.md`.