Add param to debug PPR skeleton in dev (#62703)
This adds an experimental query `__nextppronly` to allow debugging PPR
skeletons in development to avoid having to do numerous builds to be
able to debug this experience.
x-ref: [slack
thread](https://vercel.slack.com/archives/C05KYT5S9FF/p1709151588583179?thread_ts=1708474869.960689&cid=C05KYT5S9FF)
