Turbopack: Enable server HMR by default for app pages (#91476)
This enables server HMR (formally, this is called server fast refresh)
by default for app router pages. For now, it's still disabled for routes
and middleware even with app router.
- Replaces `--experimental-server-fast-refresh` with
`--no-server-fast-refresh` to opt out
- Updates telemetry to record `feature.serverFastRefreshDisabled` when
the disable flag is passed
- [x] Verify `next dev` enables server HMR by default (Turbopack, app
dir, non-edge routes)
- [x] Verify `next dev --no-server-fast-refresh` disables it
- CI
