C++ Cleanup 1/N: Reorganize YGStyle (#1349)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1349

X-link: https://github.com/facebook/react-native/pull/39171

## This diff

This diff adds a `style` directory for code related to storing and manipulating styles. `YGStyle`, which is not a public API, is renamed to `yoga::Style` and moved into this folder, alongside `CompactValue`. We will eventually add `ValuePool` alongside this for the next generation style representation.

## This stack

The organization of the C++ internals of Yoga are in need of attention.
1. Some of the C++ internals are namespaced, but others not.
2. Some of the namespaces include `detail`, but are meant to be used outside of the translation unit (FB Clang Tidy rules warn on any usage of these)
2. Most of the files are in a flat hierarchy, except for event tracing in its own folder
3. Some files and functions begin with YG, others don’t
4. Some functions are uppercase, others are not
5. Almost all of the interesting logic is in Yoga.cpp, and the file is too large to reason about
6. There are multiple grab bag files where folks put random functions they need in (Utils, BitUtils, Yoga-Internal.h)
7. There is no clear indication from file structure or type naming what is private vs not
8. Handles like `YGNodeRef` and `YGConfigRef` can be used to access internals just by importing headers

This stack does some much needed spring cleaning:
1. All non-public headers and C++ implementation details are in separate folders from the root level `yoga`. This will give us room to split up logic and add more files without too large a flat hierarchy
3. All private C++ internals are under the `facebook::yoga` namespace. Details namespaces are only ever used within the same header, as they are intended
4. Utils files are split
5. Most C++ internals drop the YG prefix
6. Most C++ internal function names are all lower camel case
7. We start to split up Yoga.cpp
8. Every header beginning with YG or at the top-level directory is public and C only, with the exception of Yoga-Internal.h which has non-public functions for bindings
9. It is not possible to use private APIs without static casting handles to internal classes

This will give us more leeway to continue splitting monolithic files, and consistent guidelines for style in new files as well.

These changes should not be breaking to any project using only public Yoga headers. This includes every usage of Yoga in fbsource except for RN Fabric which is currently tied to internals. This refactor should make that boundary clearer.

Changelog: [Internal]

Reviewed By: rshest

Differential Revision: D48710084

fbshipit-source-id: 20961aee30d54a6b0d8c1cc2976df09b9b6d486a

Yoga.podspec

@@ -35,7 +35,14 @@ Pod::Spec.new do |spec|
       '-std=c++17',
       '-fPIC'
   ]
-  spec.source_files = 'yoga/**/*.{h,cpp}'
-  spec.public_header_files = 'yoga/{Yoga,YGEnums,YGMacros,YGValue}.h'
+
   spec.swift_version = '5.1'
+  spec.source_files = 'yoga/**/*.{h,cpp}'
+  spec.header_mappings_dir = 'yoga'
+
+  public_header_files = 'yoga/{Yoga,YGEnums,YGMacros,YGValue}.h'
+  spec.public_header_files = public_header_files
+
+  all_header_files = 'yoga/**/*.h'
+  spec.private_header_files = Dir.glob(all_header_files) - Dir.glob(public_header_files)
 end