C++ style enums 9/N: FlexDirection (#1394)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1394

X-link: https://github.com/facebook/react-native/pull/39484

Moves internal usages of YGDirection to Direction.

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Changelog: [Internal]

Reviewed By: rshest

Differential Revision: D49336066

fbshipit-source-id: b49e1ffcd79a427e36ea8d2c26debaa98ef3e4c9

Yoga.podspec

@@ -46,4 +46,5 @@ Pod::Spec.new do |spec|
 
   all_header_files = 'yoga/**/*.h'
   spec.private_header_files = Dir.glob(all_header_files) - Dir.glob(public_header_files)
+  spec.preserve_paths = [all_header_files]
 end