Standardize "extern" decls on AS_EXTERN

[Adlai-Holler]

• First of a series of header/macro cleanups.
• Need 1 "extern" decorator to rule them all.
• EXTERN_C_BEGIN/EXTERN_C_END aren't sufficient because outside of c++ they don't do anything. In C you cannot declare multiple functions extern in one shot. You must decorate each function anyway, so why bother with these?
• Foundation uses FOUNDATION_EXTERN, UIKit uses UIKIT_EXTERN on each function, and drops the BEGIN/END.
• #define AS_EXTERN FOUNDATION_EXTERN
• We don't need to bother with EXPORT because that only matters when compiling for win32. Not aware of any intentions to port to windows 😂 😂 😂 🔥🔥🔥🔥🔥🔥🔥🔥🔥🔥

The current definition of FOUNDATION_EXTERN is:

#if defined(__cplusplus)
#define FOUNDATION_EXTERN extern "C"
#else
#define FOUNDATION_EXTERN extern
#endif

[maicki]

LGTM

[ghost]

	1 Warning
⚠️	This is a big PR, please consider splitting it up to ease code review.

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