pkg/macos: clean up for Zig 0.14, consolidate C imports into one decl

Fixes #6727

The major change in this commit is to consolidate all the C imports in
a single decl in main.zig. This is required for Zig 0.14. Without it,
the problem in #6727 will happen. I was never able to minimize why this
happens in order to open a Zig bug.

Beyond this, I fixed the build.zig and build.zig.zon to work with Zig
0.14 so that we can test building `pkg/macos` in isolation. There are no
downstream impacting changes in the build.zig files.

pkg/macos/foundation/attributed_string.zig

@@ -67,7 +67,7 @@ pub const MutableAttributedString = opaque {
     ) void {
         const T = @TypeOf(key);
         const info = @typeInfo(T);
-        const Key = if (info != .Pointer) T else info.Pointer.child;
+        const Key = if (info != .pointer) T else info.pointer.child;
         const key_arg = if (@hasDecl(Key, "key"))
             key.key()
         else