ci(release): create releases semantic-release

- create new release on push on `main` branch
- create new `testing` prerelease on push on `testing` branch

* .gitlab-ci.yml: new stage `release` with `new release` and
  `testing prerelease` jobs.

* .releaserc.js: configuration file for `semantic-release`.

* .eslintignore: ignore the `semantic-release` configuration and
  `dist/` generated file.