Migrate from Mocha/Sinon/Chai to Vitest

[Kocal]

Purely an internal subject, but I would like to modernize our tests and use 1 single tool instead of 3 different tools (less frictions, easier to update, ...).

Since Vitest is based on Vite, it may helps in the future:

• to ship an ESM version (like proposed in [RFC] Migrate webpack-encore to ESM #1312) without dropping the CJS version (which won't requires a new major version
• to rewrite the code base in TypeScript