hash-lang: Add region behavior to automatic pairs

Similar to electric-pairs-mode, when a region is active, wrap it with
the pair.

Although I don't want to go too far down the road on reinvented
wheels, this seems like a basic nice thing to add without too much
effort.

racket-hash-lang.el

@@ -249,6 +249,7 @@ can contribute more colors; see the customization variable
               (append (list (cons 'racket-token t))
                       text-property-default-nonsticky))
   (add-hook 'post-self-insert-hook #'racket-hash-lang-post-self-insert nil t)
+  (add-hook 'self-insert-uses-region-functions #'racket-hash-lang-will-use-region nil t)
   (electric-pair-mode -1)
   (electric-indent-local-mode -1)
   (setq-local electric-indent-inhibit t)
@@ -609,18 +610,39 @@ You may customize this default initialization in
               vs)))
     (setq-local racket-hash-lang-pairs (reverse vs))))
 
+(defun racket-hash-lang-will-use-region ()
+  "A value for hook `self-insert-uses-region-functions'."
+  (and (use-region-p)
+       (assq last-command-event racket-hash-lang-pairs)))
+
 (defun racket-hash-lang-post-self-insert ()
-  "A value for `post-self-insert-hook'."
-  (cl-flet ((self-insert (char)
-                         (let ((racket-hash-lang-pairs nil) ;don't recur!
-                               (last-command-event char)
-                               (blink-matching-paren nil))
-                           (self-insert-command 1)
-                           (forward-char -1))))
+  "A value for hook `post-self-insert-hook'."
+  (cl-flet* ((self-insert
+              (char)
+              (let ((racket-hash-lang-pairs nil) ;don't recur!
+                    (blink-matching-paren nil)
+                    (last-command-event char))
+                (self-insert-command 1)))
+             (add-close
+              (open-char close-char)
+              (if (use-region-p)
+                  (if (<= (point) (mark))
+                      (save-excursion
+                        (goto-char (mark))
+                        (self-insert close-char))
+                    (save-excursion
+                      (let ((end (point)))
+                        (delete-char -1) ;delete open at end
+                        (goto-char (mark))
+                        (self-insert open-char)
+                        (goto-char end)
+                        (self-insert close-char))))
+                (save-excursion
+                  (self-insert close-char)))))
     (pcase (assq last-command-event racket-hash-lang-pairs)
-      (`(,_open ,close)
-       (self-insert close))
-      (`(,_open ,close . ,except-kinds)
+      (`(,open ,close)
+       (add-close open close))
+      (`(,open ,close . ,except-kinds)
        (pcase-let ((`(,_beg ,_end (,kind . ,_))
                     (racket--cmd/await
                      nil
@@ -630,7 +652,7 @@ You may customize this default initialization in
                        ,racket--hash-lang-generation
                        ,(1- (point))))))
          (unless (memq kind except-kinds)
-           (self-insert close)))))))
+           (add-close open close)))))))
 
 (defun racket-hash-lang-delete-backward-char ()
   "Delete previous character, and when between a pair, following character."