fs: fix recursive readdir with buffer encoding

[canblmz1]

Description

fs.readdir(), fs.readdirSync(), and fs.promises.readdir() throw
ERR_INVALID_ARG_TYPE (or crash the process outright in the callback
case) when called with both { recursive: true } and
{ encoding: 'buffer' }. The internal recursive walk builds paths with
path.join()/path.relative(), and checks whether an entry is a
directory via a CommonJS-module-resolution-specific native stat
binding (internalModuleStat) - none of these accept Buffer
arguments, which is what entry names become once encoding: 'buffer'
is set.

const fs = require('fs');
fs.readdirSync('.', { recursive: true, encoding: 'buffer' });
// Uncaught TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument
// must be of type string. Received an instance of Buffer

Fix

• internal/fs/utils already had an unexported join() helper that
  correctly handles string/Buffer combinations; this exports it and
  adds two more helpers alongside it: relativeToBasePath() (avoids
  needing path.relative()'s Buffer support, since the full path is
  always built by repeatedly joining onto the base path) and
  isDirectoryPath().
• isDirectoryPath() keeps using internalModuleStat for the string
  fast path (unchanged behavior/performance for the common case), but
  falls back to the general-purpose stat binding - the same one
  fs.statSync() uses - for Buffer paths. That binding handles
  Buffers correctly at the native layer, so this also avoids a lossy
  string round-trip for non-UTF8 file names, rather than just papering
  over the reported crash.
• lib/fs.js (handleDirents/handleFilePaths) and
  lib/internal/fs/promises.js (readdirRecursive) both had their
  own copy of this bug and are updated to use the shared helpers.

Testing

Added test/parallel/test-fs-readdir-recursive-buffer.js, covering
readdirSync, readdirSync with withFileTypes, the readdir
callback form, and fs.promises.readdir, all with
{ recursive: true, encoding: 'buffer' }.

Ran the full test/parallel/test-fs-readdir*.js suite plus a broader
test/parallel/test-fs-*.js sweep (257 files) locally on Windows
(clang-cl build) - all pass except 7 pre-existing failures unrelated
to this change (EPERM on symlinkSync due to this environment not
running elevated/without Developer Mode, verified individually).

Fixes: #58892