Add Directory.copy (and Directory.copySync)

[tvolkert]

Something along the lines of this:

https://github.com/flutter/flutter/blob/ee97e5639ec704df4b5179cc88ebc19f0de9d8bd/packages/flutter_tools/lib/src/base/file_system.dart#L35

(originally filed by @xster)

[sgehrman]

found this: https://pub.dev/documentation/io/latest/io/copyPathSync.html