chore(eslint): migrate from eslint-plugin-vitest to @vitest/eslint-plugin and run rules only on test files

[KristjanESPERANTO]

This PR makes three small changes to the ESLint setup:

1. Migrate from eslint-plugin-vitest to it's successor @vitest/eslint-plugin.
2. Change the eslint config so that only the test files are checked with the vitest rules. Previously, it was just unnecessary and inefficient to check all js files with them.
3. We had defined some of the test rules as warnings, but that is not really ideal. I changed them to errors.