add tracing for turbopack (#49920)
### What?
adds `NEXT_TURBOPACK_TRACING` env var to enable tracing. Writes into `.next/trace.log`
There are 4 presets:
* `NEXT_TURBOPACK_TRACING=overview` gives a overview of requests and modules processed.
* `NEXT_TURBOPACK_TRACING=next` above plus details for next.js
* `NEXT_TURBOPACK_TRACING=turbopack` above plus details for turbopack
* `NEXT_TURBOPACK_TRACING=turbo-tasks` above plus details for turbo-tasks
Published release builds will only allow `overview` to work, since all detailed instrumentation is statically disabled.
see https://github.com/vercel/turbo/pull/4966 for more details
### Why?
get more insight into build times
### Turbopack changes
* https://github.com/vercel/turbo/pull/4995
* https://github.com/vercel/turbo/pull/5049
* https://github.com/vercel/turbo/pull/5053
* https://github.com/vercel/turbo/pull/4966
