Alternate bundler: show state in app info message (#77259)
When next starts, in either `next dev` or `next start`, we log a message
that includes Turbopack state (`▲ Next.js 15.3.0-canary.12
(Turbopack)`). This implements it for rspack as well.
Test Plan:
Configure the rspack plugin and start `next dev`. Confirm `▲ Next.js
15.3.0-canary.12 (rspack)` is shown.
Run `next dev --turbopack`. Verify Turbopack is shown instead.
