win,fs: use CreateDirectoryW instead of _wmkdir
win,fs: use CreateDirectoryW instead of _wmkdir

No functional changes are intended [NFCI], but this may make it easier
in the future to implement and respect the `modes` flag.

PR-URL: https://github.com/libuv/libuv/pull/2921
Reviewed-By: Jameson Nash <99e7f19e9f64df56d4e81dc00b8566e722cdd766@gmail.com>
unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso