Telemetry: include alternate bundler state in key events (#76886)
This includes `isRspack` in the following telemetry events, determining
it with the presence of `process.env.NEXT_RSPACK`:
- `NEXT_CLI_SESSION_STARTED`
- `NEXT_CLI_SESSION_STOPPED`
- `NEXT_BUILD_OPTIMIZED`
Test Plan: Verify these are included with `NEXT_TELEMETRY_DEBUG=1` for
both `next dev` and `next build`