chore: upgrade Vue to version 3

This MR upgrades the build tooling and dependencies from Vue2 to Vue3, whilst still keeping (mostly) the same prod code.

1. I used the npm init vue@latest to create a new Vue3 application.
2. Then I installed the Vue Migration Build to have compatibility with Vue2.
3. I installed vitest and changed all Jest-specific parts of the tests to Vitest-specific.

Closes gitlab#416390 (closed)