refactor!: deprecate IS-* OS constants

BREAKING CHANGE: This deprecates the IS-(MAC|WINDOWS|LINUX|BSD) family
of global constants in favor of a native `featurep` check:

  IS-MAC      ->  (featurep :system 'macos)
  IS-WINDOWS  ->  (featurep :system 'windows)
  IS-LINUX    ->  (featurep :system 'linux)
  IS-BSD      ->  (featurep :system 'bsd)

The constants will stick around until the v3 release so folks can still
use it -- and there are still some modules that use it, but I'll phase
those uses out gradually.

Fix: #7479

modules/app/everywhere/doctor.el

@@ -1,9 +1,9 @@
 ;;; app/everywhere/doctor.el -*- lexical-binding: t; -*-
 
-(when IS-WINDOWS
+(when (featurep :system 'windows)
   (error! "emacs-everywhere package does not support windows."))
 
-(when IS-LINUX
+(when (featurep :system 'linux)
   (let (unmet-deps)
     (dolist (dep '("xclip" "xdotool" "xprop" "xwininfo"))
       (unless (executable-find dep)

modules/app/irc/config.el

@@ -187,8 +187,8 @@ playback.")
   (setq circe-notifications-watch-strings +irc-notifications-watch-strings
         circe-notifications-emacs-focused nil
         circe-notifications-alert-style
-        (cond (IS-MAC 'osx-notifier)
-              (IS-LINUX 'libnotify)
+        (cond ((featurep :system 'macos) 'osx-notifier)
+              ((featurep :system 'linux) 'libnotify)
               (circe-notifications-alert-style))))