fs: default open/openSync flags argument to 'r'

Make fs.open() and fs.openSync() more economic to use by making the
flags argument optional. You can now write:

    fs.open(file, cb)

Instead of the more verbose:

    fs.open(file, 'r', cb)

This idiom is already supported by functions like fs.readFile().

PR-URL: https://github.com/nodejs/node/pull/23767


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>