Add a way to debug `_payload.json`

[Barbapapazes]

Hello,

In Nuxt3, all useFetch or useAsyncData push content in a _payload file when page is prerendered. This is cool but it's pretty easy to have some heavy payloads because of a mistake like keeping too many keys with Content.

Could be nice to introduce a new command to easily view every payload size.

[pi0]

(discussion from #258 (comment), to move it to a standalone package to iterate)