fix(ext/node): apply encoding to Dirent name/parentPath in fs.readdir (#33972)
- When `fs.readdir`/`readdirSync` is called with `{ withFileTypes: true,
encoding: 'buffer' }`, Node returns `Dirent` instances whose `name` and
`parentPath` fields are `Buffer` instances. Deno was ignoring the
`encoding` option on the `withFileTypes` code path and returning
strings.
- Apply the requested encoding to both fields via the existing
`decode()` helper, matching Node's behavior for `'buffer'` and other
non-utf8 encodings.
Fixes #33957