Fix visit docs for Racket & support racket-xp (#3373)

* Fix visit docs for Racket & support racket-xp

- Fix Racket's +lookup/documentation in the same way as
  f20f477a44
- Support the +xp feature which enables racket-xp. When it's not
  enabled, default handlers to the traditional racket-repl
  (which requires users to run code first)

* PR feedback

* Replace add-hook! with add-hook

modules/lang/racket/autoload.el

@@ -10,3 +10,20 @@
               (let ((buf (get-buffer "*Racket REPL*")))
                 (bury-buffer buf)
                 buf)))))
+
+;;;###autoload
+(defun +racket-lookup-documentation (thing)
+  "A `+lookup/documentation' handler for Racket."
+  (let ((buf (if racket-xp-mode
+                 (racket-xp-describe thing)
+               (racket-repl-describe thing))))
+    (when buf
+      (pop-to-buffer buf)
+      t)))
+
+;;;###autoload
+(defun +racket-lookup-definition (_thing)
+  "A `+lookup/definition' handler for Racket."
+  (if racket-xp-mode
+      (call-interactively #'racket-xp-visit-definition)
+    (call-interactively #'racket-repl-visit-definition)))