introduce RCTFoundation (#41418)

Summary:

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

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

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+# 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.