pair @vitest/coverage-v8 with the vitest 5 major

Dependabot merged the vitest ^5.0.0 major (#39) without its
coverage-v8 peer, leaving main's npm install ERESOLVE-broken. This
bumps @vitest/coverage-v8 to the matching ^5.0.0 - install resolves
clean (deduped 5.0.0 pair) and the suite matches pristine main
(237 passed; the 11 cli failures are the known local-env artifact,
CI-green on main).

Author: ronaldtse