React-jsidynamic: Move JSIDynamic out of React-jsi

Summary:
The jsi library itself is `jsi/jsi.{h,cpp}`. JSIDynamic provides support for converting between folly::dynamic and jsi::value, independent of the jsi library.

Changelog:
[iOS][Changed] Moved JSIDynamic out of React-jsi and into React-jsidynamic

Reviewed By: cipolleschi, dmytrorykun

Differential Revision: D40334023

fbshipit-source-id: d2c69e7afb7f43f93080301b88c81e1fa46279d7

ReactCommon/ReactCommon.podspec

@@ -54,6 +54,7 @@ Pod::Spec.new do |s|
     ss.subspec "core" do |sss|
       sss.source_files = "react/nativemodule/core/ReactCommon/**/*.{cpp,h}",
                          "react/nativemodule/core/platform/ios/**/*.{mm,cpp,h}"
+      sss.dependency "React-jsidynamic", version
     end
 
     s.subspec "react_debug_core" do |sss|