What problem does this feature solve?

sometime ssr failed means there's something wrong with the code, if we ignore the ssg failed and fallback to csr which may cause csr fail too, which happens in web-infra-dev/rspack#7505

What does the proposed API look like?

add a option to disable ssg fallback

{
  fallback: false | 'csr' // false mean don't do fallback when ssg failed , 'csr' means fallback to csr when ssg failed
}