Use vitest instead of jest for frontend addon tests.

[davisagli]

In #165 @sneridagh said:

we will move away from Jest soon enough. There's a very advanced PR in the pipeline which will replace it with vitest. If you are starting a new project, I can't recommend enough that you just use it. The setup is seamless, it just works with the current ecosystem.

@sneridagh Where can we find an example of the vitest setup so we can figure out how to update the cookieplone templates? Will this change be compatible with existing versions of Volto or is it only for the future?

[sneridagh]

@davisagli we have two options in here with plone/volto#6605:

1- We make it a breaking, and we update the cookieplone template to use it (in an 19 alpha). We remove all traces of Jest from core.
2- We make it backwards compatible, so people can still use their Jest tests with core, by leaving the Jest config in there, but core uses vitest. In this option people optionally can migrate their setups to vitest.
3- We go 2, but new cookieplone uses vitest from now on. This has the drawback that you could only use it from one 18 version.

I think I'd go for 2.

The tests are quite easy to migrate since the code is almost 1:1, we can ask the PR author to write a migration guide that covers all the bases. The boilerplate is a bit different, but quite reduced (vitest.config.ts, almost zero config based, one needs to add the Volto specific artifacts)