refactor: deprecate EMACS2[89]+, NATIVECOMP, MODULES

To reduce redundancy, remove the maintenance hassle that version
constants would impose later on, and rely on built-in
facilities (featurep) more over global variables or doomisms, these
global constants have been deprecated in favor of Emacs "features":

- EMACS28+   -- replace with (> emacs-major-version 27)
- EMACS29+   -- replace with (> emacs-major-version 28)
- NATIVECOMP -- replace with (featurep 'native-compile)
- MODULES    -- replace with (featurep 'dynamic-modules)

(These constants will be formally removed when v3 is released. The IS-*
constants are likely next, but I haven't decided on their substitutes
yet)

I also decided to follow native-compile's example and provide features
for Emacs' system features (since system-configuration-features' docs
outs itself as a poor method to detect features):

- dynamic-modules
- jansson
- native-compile -- this one already exists, but will instead be removed
  if it's non-functional; i.e. (native-comp-available-p) returns nil.

These are now detectable using featurep, which is fast and built-in.

lisp/cli/doctor.el

@@ -67,7 +67,7 @@ in."
                "\t- might depend subtly on upstream packages updates.\n"
                "You might need to unpin packages to get a fix for a specific commit of Emacs, "
                "and you should be ready to downgrade Emacs if something is just not fixable."))
-    (EMACS29+
+    ((> emacs-major-version 28)
      (warn! "Emacs %s detected" emacs-version)
      (explain! "Doom supports this version, but you are living on the edge! "
                "Be prepared for breakages in future versions of Emacs."))
@@ -108,13 +108,12 @@ in."
               "jansson support (i.e. a native JSON library), particularly LSP users. "
               "You must install a prebuilt Emacs binary with this included, or compile "
               "Emacs with the --with-json option."))
-  (when EMACS28+
-    (unless NATIVECOMP
-      (warn! "Emacs was not built with native compilation support")
-      (explain! "Users will see a substantial performance gain by building Emacs with "
-                "native compilation support, availible in emacs 28+."
-                "You must install a prebuilt Emacs binary with this included, or compile "
-                "Emacs with the --with-native-compilation option.")))
+  (unless (featurep 'native-compile)
+    (warn! "Emacs was not built with native compilation support")
+    (explain! "Users will see a substantial performance gain by building Emacs with "
+              "native compilation support, availible in emacs 28+."
+              "You must install a prebuilt Emacs binary with this included, or compile "
+              "Emacs with the --with-native-compilation option."))
 
   (print! (start "Checking for private config conflicts..."))
   (let* ((xdg-dir (concat (or (getenv "XDG_CONFIG_HOME")