Platform Commonalities: e.g. Apple, POSIX, etc.

Hi wonderful Bazel Platforms folks!

First and foremost, thanks for working to create something centralized and great here. It's so nice to have official support for modeling cross-platform code with platform-specific backends (among other goodies).

I'd like to float an idea for making official something I find myself rebuilding pretty much every time I use platforms.

Lots of platforms (err constraint_values) have considerable code in common. It can be nice to select on this commonality. Some examples are: Apple Platforms. POSIX/UNIX-compliant platforms. Etc.

It's not hard to model these (example below), but it'd be awesome to have them in the @platforms repo for everyone to use!
```
load("@bazel_skylib//lib:selects.bzl", "selects")
selects.config_setting_group(
name = "Apple",
visibility = ["//visibility:public"],
match_any = ["@platforms//os:macos", "@platforms//os:ios", "@platforms//os:tvos", "@platforms//os:watchos"],
)
```

Happy to contribute of course--and would have made this a PR--except that I'm not sure quite how this repo is being automagically integrated into Bazel proper. Therefore I wasn't sure how to properly get skylib.

Thanks again, additionally for your consideration!
Chris
(ex-Googler)