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

lisp/lib/system.el

@@ -4,8 +4,8 @@
 (defun doom-system-distro ()
   "Return a symbol representing the installed distro."
   (with-memoization (get 'doom-system-distro 'cached-value)
-    (cond (IS-WINDOWS 'windows)
-          (IS-MAC     'macos)
+    (cond (doom--system-windows-p 'windows)
+          (doom--system-macos-p     'macos)
           ((ignore-errors
              (with-file-contents! "/etc/os-release"
                (when (re-search-forward "^ID=\"?\\([^\"\n]+\\)\"?" nil t)