Try to setup vitest integration

[Jym77]

• Build breaks because Vitest depends on some DOM global. Adding lib: ["DOM"] to compiler optionsis a workaround but it is a bit overkill. Adding skipLibCheck is going to skip all libraries and is also overkill. See vite config files can't be compiled without dom types vitejs/vite#9813
• Build breaks because Vitest duplicates Chai types, thus conflicting with Chai's own definitions imported for alfa-chai. See @types/chai conflict with @vitest/expect/dist/chai.d.cts vitest-dev/vitest#4688

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