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.

modules/term/eshell/autoload/backport.el

@@ -1,5 +1,5 @@
 ;;; term/eshell/autoload/mode.el -*- lexical-binding: t; -*-
-;;;###if (not EMACS28+)
+;;;###if (< emacs-major-version 28)
 
 ;; DEPRECATED Remove this when we drop Emacs 27 support.
 

modules/term/vterm/config.el

@@ -1,7 +1,7 @@
 ;;; term/vterm/config.el -*- lexical-binding: t; -*-
 
 (use-package! vterm
-  :when (bound-and-true-p module-file-suffix)
+  :when (featurep 'dynamic-modules)
   :commands vterm-mode
   :hook (vterm-mode . doom-mark-buffer-as-real-h)
   :hook (vterm-mode . hide-mode-line-mode) ; modeline serves no purpose in vterm