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/doom-editor.el

@@ -561,7 +561,7 @@ files, so this replace calls to `pp' with the much faster `prin1'."
        (lambda (button)
          (helpful-variable (button-get button 'apropos-symbol))))))
 
-  (when EMACS29+
+  (when (> emacs-major-version 28)
     ;; REVIEW This should be reported upstream to Emacs.
     (defadvice! doom--find-function-search-for-symbol-save-excursion-a (fn &rest args)
       "Suppress cursor movement by `find-function-search-for-symbol'.
@@ -669,7 +669,7 @@ on."
   ;; Emacs 29 introduced faster long-line detection, so they can afford a much
   ;; larger `so-long-threshold' and its default `so-long-predicate'.
   (if (fboundp 'buffer-line-statistics)
-      (unless NATIVECOMP
+      (unless (featurep 'native-compile)
         (setq so-long-threshold 5000))
     ;; reduce false positives w/ larger threshold
     (setq so-long-threshold 400)