Support "browser-mode" in the vitest runner

[nicojs]

https://vitest.dev/guide/browser.html

... or at least crash with a nice error message.

EDIT: We probably want to implement this later, see vitest-dev/vitest#3447. For now an error message is fine.

[nicojs]

Not sure if we can make it to milestone 7.0. We might be able to run without coverage analysis ('off')