Use node: protocol for all builtin imports (fs, path, etc.)

[nolanlawson]

We should use an ESLint plugin like prefer-node-protocol and ensure we're always doing this:

import fs from 'node:fs'

instead of this:

import fs from 'fs'

The node: protocol is clearer, more portable, and less likely to conflict with any accidental non-builtins (e.g. assert).

This has some mild Node compat issues, but since we probably only support Node 16+ anyway, we can probably safely do this. (Or we can do this in a major version bump and then be explicit about our supported Node version.)

[nolanlawson]

Removed up for grabs because this is a breaking change so probably requires a lot of care and thought.