chore(apple): fix build issues caught internally

[Saadnajmi]

Summary

At my side of Microsoft, we have recently "vendored" a couple of libraries. Which means, we started building them from source using our own generated Xcode project (not the one cocoa pods generates), which has much stricter build flags / treats warnings as errors. In doing so, I caught a couple of issues in React Native SVG I'd like to push back up!

Namely:

• Fix a bunch of errors where "declaration of local variable shadows a previous declaration" by renaming some variables
• Add __unused clang pragmas in a couple of places
• Move some enums from CF_ENUM to NS_ENUM, and include an import of <Foundation.h>
• cast some NSIntegers to intss or longs depending on context
• Fix some headers by changing them from Header Search Paths (<Foo.h>) to User Header Search Paths ("foo.h")
• Rename "self" to "defaults" inside RNSVGFontData to fix a "Declaration shadows a local variable" (I guess self is always in scope?)
• Import <TargetConditionals.h> in places where we use #if TARGET_OS_OSX ifdefs
• Also ifdef out some iOS files on macOS, and vice versa. Specifically, RNSVGTopAlignedLabel
• In RNSVGSpan, move the label object from declared in a random spot in the file (??!) to be a proper ivar... this one was fun to catch.
• Import RNSVGUIKit in one spott where it's used but not referenced.
• Move the union kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big; to a separate variable. Technically, those two types are actually values from two different enums, but they are meant to be used together and cast, so 🤷‍♂️

Test Plan

CI should pass at the very least

What's required for testing (prerequisites)?

What are the steps to reproduce (after prerequisites)?

Compatibility

OS	Implemented
iOS	✅❌
MacOS	✅❌
Android	✅❌
Web	✅❌

Checklist

• I have tested this on a device and a simulator
• I added documentation in README.md
• I updated the typed files (typescript)
• I added a test for the API in the __tests__ folder