Add tests and automated publishing the package to NPM

[robmosca]

This PR:

• introduces automated publishing of the package using the changeset action. The setup is identical to the one in single-spa-react
• introduces Jest tests on the API

For automated publishing to work, it is necessary that:

• The GITHUB_TOKEN is granted permissions to create and approve pull requests. This can be done in Settings -> Actions -> General -> Workflow permissions -> Allow GitHub Actions to create and approve pull requests
• The admin of the repo creates an NPM_TOKEN secret with an NPM token with write permissions to the organization (or at least the corresponding package, for publishing).

[changeset-bot]

🦋 Changeset detected

Latest commit: 09057fe

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
import-map-overrides	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[robmosca]

Closed in favour of #96