src,lib: expose memory file mapping flag

Support for UV_FS_O_FILEMAP was added in libuv in version 1.31.0.
This exposes the flag in fs.constants.

Refs: https://github.com/libuv/libuv/pull/2295
PR-URL: https://github.com/nodejs/node/pull/29260


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ron Korving <ron@ronkorving.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>