Setup Backend Testing

## Basic Information
Setup [Vitest](https://vitest.dev/) to provide unit & integration tests for the backend.
In addition, add a job to the `javascript-ci.yml` workflow to run the tests on relevant PRs.

## Other Information
The tests should use the [Firebase local emulator](https://firebase.google.com/docs/emulator-suite).

See [GitHub Actions' documentation](https://docs.github.com/en/actions) for guidance on writing workflows.
As for determining relevancy, the [`paths:`](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onpushpull_requestpull_request_targetpathspaths-ignore) key can be used to check for changes in specific directories before running.