Write out task statistics if NEXT_TURBOPACK_TASK_STATISTICS is set (#67164)
Writes out the statistics gathered via vercel/turbo#8286 to a file.
Stats can be formatted and sorted with jq:
```
jq 'to_entries | sort_by(.value.cache_miss) | reverse | from_entries' <input.json >output.json
```
Output looks something like this (once formatted and sorted):
https://gist.github.com/bgw/4e2df35b9e410bf71fe51ecaffc3c7bf
Without #67165, this requires that SIGINT be sent directly to the
`next-server` process to allow a clean exit:
```
pkill -INT next-server
```
But with #67165, a simple <kbd>ctrl</kbd>+<kbd>c</kbd> works.