fs.readdirSync on a file: ENOTDIR on Windows, not EINVAL - #91

What

tslang_fs_read_dir (src/wrappers/os.cpp) now reports ENOTDIR when the path is an existing file that isn't a directory, as Node does.

Why

On Windows, opening a directory iterator on a file fails with ERROR_DIRECTORY, which MSVC's std::system_category maps to EINVAL. tests/fs.ts expects ENOTDIR (or ENOENT) in its "list a file" check, so it failed on every Windows job (EINVAL: invalid argument, scandir '...'). Linux already reports ENOTDIR, so nothing changes there.

Notes for the reviewer

• Tested locally on Windows, release, -mm=gc, both compiled and under the JIT: fs, process, process_exit, array_flat, map_groupBy and string_fromCodePoint all print ALL DONE.
• Not run locally: the debug pass (this machine's compiler debug runtime is out of date) and the rc/none/own models on Windows. CI covers those.
• The Windows jobs on process and fs; Array flat/flatMap/toSpliced, Map.groupBy, String.fromCodePoint #90 ran for over an hour instead of failing. My guess, not confirmed: in the debug pass the failed assert calls abort(), which opens a modal dialog that nobody closes. This fix stops that assert from failing.

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