Predefined Versions for Apple Operating Systems

[Walter Bright]

Currently, the following predefined versions exist for Apple operating systems:

OSX iOS TVOS WatchOS VisionOS

When I look at druntime source code, I see things like:

```d
version (OSX)
     version = Darwin;
else version (iOS)
     version = Darwin;
else version (TVOS)
     version = Darwin;
else version (WatchOS)
     version = Darwin;
```
and:
```d

```d
version (OSX) {}
else version (iOS) {}
else version (TVOS) {}
else version (WatchOS) {}
else:
```
appearing ad nauseam.

(note the conspicuous absence of VisionOS appearing in druntime)

I propose, like Windows encompassing Win32 and Win64, that __APPLE__ encompass 
OSX, iOS, TVOS, WatchOS, and VisionOS. __APPLE__ is already standardized in C 
headers for that purpose.

The use of "darwin" or "Darwin" appears to be quite obsolete.

This will clean up a lot of version algebra in the druntime files.