Support node:fs/promises

[dario-piotrowicz]

One common need that Next.js applications have is that of interacting with the filesystem.

One particularly common use case even mentioned in the Next.js docs is that of iterating over a collection of MDX and extract data, for example to create a glob index page (ref: #261).

So in order to support that (at runtime, with ISR) we need to have some sort of filesystem's support implemented using some storage solution such as KV or R2.

Since sync fs operations would require very involved changes in our runtime, it's probably enough to just aim to support async fs operations.