build: update Vitest and Sveltkit Alias configs

[kaljarv]

We're using an old version of vitest and it should be updated.

Also, check that the Sveltekit Aliases are configured correctly. See:

• https://github.com/sveltejs/kit/issues/5962
• https://github.com/OpenVAA/voting-advice-application/commit/339ecb160afdbf728cfe7e11ba95eec77824de51