feat(lib): add doom-font-exists-p

For easier font detection in your config.

Ref: #5948

core/autoload/fonts.el

@@ -109,6 +109,12 @@ Doesn't work in terminal Emacs."
      (ignore-errors (doom-adjust-font-size nil))
      (signal (car e) (cdr e)))))
 
+;;;###autoload
+(defun doom-font-exists-p (font)
+  "Return non-nil if FONT exists on this system."
+  (declare (pure t) (side-effect-free t))
+  (ignore-errors (find-font (doom-normalize-font font))))
+
 
 ;;
 ;;; Commands