Added Vitest as test runner

[cawa-93]

I added a Vitest and wrote some additional unit-tests.

Vitest is still in development and is not recommended for production. However, I did this as an example of integrating the Vitest with this project. You can always replace it with any test-framework.

Also was added 2 dev dependencies:

• happy-dom -- For testing UI component. May be useful for everybody.
• @vue/test-utils@next -- Unit testing utility library for Vue.js 3. Will be useful for Vue projects.