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/ui/modeline/+light.el

@@ -489,7 +489,7 @@ lines are selected, or the NxM dimensions of a block selection.")
   `(:eval
     (let ((sys (coding-system-plist buffer-file-coding-system))
           (eol (coding-system-eol-type-mnemonic buffer-file-coding-system)))
-      (concat (unless (equal eol ,(if IS-WINDOWS "CRLF" "LF"))
+      (concat (unless (equal eol ,(if (featurep :system 'windows) "CRLF" "LF"))
                 (concat "  " eol " "))
               (if (memq (plist-get sys :category)
                         '(coding-category-undecided coding-category-utf-8))

modules/ui/modeline/config.el

@@ -24,9 +24,7 @@
         ;; than the current OSes preference
         doom-modeline-buffer-encoding 'nondefault
         doom-modeline-default-eol-type
-        (cond (IS-MAC 2)
-              (IS-WINDOWS 1)
-              (0)))
+        (pcase doom-system ('macos 2) ('windows 1) (_ 0)))
 
   :config
   ;; Fix an issue where these two variables aren't defined in TTY Emacs on MacOS