introduce RCTFoundation (#41418)

Summary:
Pull Request resolved: #41418

Changelog: [iOS][Added]

creating a top level directory for shared lightweight utility functions

open to suggs on naming and rules we want to enforce

Reviewed By: shwanton, christophpurrer

Differential Revision: D51170983

fbshipit-source-id: 8bc0a193b486f5a0653ad58d92a034cacede2d61

packages/react-native/ReactApple/Libraries/RCTFoundation/README.md

@@ -0,0 +1,12 @@
+# RCTFoundation
+
+RCTFoundation is a collection of lightweight utility libraries.
+
+Rules for RCTFoundation libraries:
+- They must only depend on other RCTFoundation libraries.
+- Headers cannot contain C++.
+- They have modular set to true in BUCK.
+- They have complete_nullability set to true.
+- They have enabled Clang compiler warnings.
+- They have documentation.
+- They have unit tests.