Initial UIKitForMac support (#25427)

Summary:
This PR adds initial support for Project Catalyst a.k.a. UIKitForMac. This is not yet meant for production, but this is enough for RNTester to successfully compile and mostly work :)

Some APIs are not supported on the Mac -- e.g. telephony, and deprecated APIs are removed on Mac ���-- those had to be ifdef'd out via platform checks.

The biggest limitation right now is that I couldn't get Web Socket code to successfully compile, and so there are a lot of temporary platform checks  for that , and the RCTWebSocket.xcodeproj is marked as not supporting UIKitForMac. Again -- temporary, until someone with more knowledge knows how to fix this.

https://github.com/react-native-community/discussions-and-proposals/issues/131

## Changelog

[iOS] [Added] - Fixed compilation for macOS (Project Catalyst) -- not meant for production use yet
Pull Request resolved: https://github.com/facebook/react-native/pull/25427

Test Plan:
- Open RNTester/RNTester.xcodeproj with Xcode 10.2, run it like a normal iOS app -- make sure it compiles and runs correctly (no regression)
- Open the same project with Xcode 11 beta 2 (or higher) on macOS Catalina beta, select "My Mac" as device target, and run -- see that it actually compiles and runs. **Note** there are unfortunately some required steps:
   - change build configuration to Release (because packager doesn't work correctly yet)
   - change development team to yours if Xcode tells you to
   - go to RNTester project → Build phases → Link binary with libraries, and change `platforms` for `libRCTWebSocket.a` to `iOS` (without Mac compatibility). I can't commit that change because it breaks compatibility with earlier Xcode versions

The two extra steps for successful compile will disappear once web socket compilation for Catalyst is fixed

Reviewed By: mmmulani

Differential Revision: D16088263

Pulled By: sammy-SC

fbshipit-source-id: 9c0b932b048e50a8e0f336eaa0612851b1909cae