Add Apple visionos.
I'm not really satisified with this PR. The number of Apple specific platforms has grown to the point where we may want to refactor them. The question I would focus on is how various toolchain matches and select clauses go.
Do we see things like:
```
foo = select({
".../os:linux": A,
".../os:windows": B,
".../os:macos": C,
".../os:watchos": C,
".../os:visionos": C,
})
```
Where C is the same for all the apple platforms?
Or, do we see real distinctions across the various per-device OSes. Or a mix of both?
And, do we see the fanout of the Apple OSes done with a select_or wrapper, so users end up seeing the simple selection of just apple, linux, or windows, but we buried complexity elsewhere?
diff --git a/os/BUILD b/os/BUILD
index e0484ee..1d9fedb 100644
--- a/os/BUILD
+++ b/os/BUILD
@@ -83,6 +83,11 @@
)
constraint_value(
+ name = "visionos",
+ constraint_setting = ":os",
+)
+
+constraint_value(
name = "qnx",
constraint_setting = ":os",
)
