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/tools/lookup/autoload/lookup.el

@@ -413,7 +413,7 @@ Otherwise, falls back on `find-file-at-point'."
              (read-string "Look up in dictionary: "))
          current-prefix-arg))
   (message "Looking up dictionary definition for %S" identifier)
-  (cond ((and IS-MAC (require 'osx-dictionary nil t))
+  (cond ((and (featurep :system 'macos) (require 'osx-dictionary nil t))
          (osx-dictionary--view-result identifier))
         ((and +lookup-dictionary-prefer-offline
               (require 'wordnut nil t))

modules/tools/lookup/config.el

@@ -216,7 +216,7 @@ Dictionary.app behind the scenes to get definitions.")
 
 (use-package! define-word
   :when (modulep! +dictionary)
-  :unless IS-MAC
+  :unless (featurep :system 'macos)
   :defer t
   :config
   (setq define-word-displayfn-alist

modules/tools/lookup/packages.el

@@ -25,7 +25,7 @@
     (package! counsel-dash :pin "8decb980f111ebe7027361ee252279a9076da261")))
 
 (when (modulep! +dictionary)
-  (if IS-MAC
+  (if (featurep :system 'macos)
       (package! osx-dictionary :pin "0715e5a3ac659df32a0f0fabfbbeef0228fbd9a9")
     (package! define-word :pin "31a8c67405afa99d0e25e7c86a4ee7ef84a808fe")
     ;; REVIEW: This fork fixes SavchenkoValeriy/emacs-powerthesaurus#40.