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/ui/ligatures/config.el

@@ -179,7 +179,7 @@ and cannot run in."
 
  ;; Harfbuzz and Mac builds do not need font-specific ligature support
  ;; if they are above emacs-27.
- ((and EMACS28+
+ ((and (> emacs-major-version 27)
        (or (featurep 'ns)
            (string-match-p "HARFBUZZ" system-configuration-features))
        (featurep 'composite))  ; Emacs loads `composite' at startup