Add tests

[Barbapapazes]

Tests must be done using vitest.

Some of them can be simple (I tried to write pure functions) but some others will required a browser environnement).

[Barbapapazes]

I think we could add e2e tests but I'm waiting for nuxt/test-utils#881 before doing some of them. I lost my mind, it has nothing related with Nuxt

I also need to retry to reuse vitest.