Make the arrow keys honor RTL.

At least to my understanding, the pages will flip in the opposite direction.

src/theme/book.js

@@ -557,21 +557,36 @@ function playground_text(playground, hidden = true) {
     document.addEventListener('keydown', function (e) {
         if (e.altKey || e.ctrlKey || e.metaKey || e.shiftKey) { return; }
         if (window.search && window.search.hasFocus()) { return; }
+        var html = document.querySelector('html');
 
-        switch (e.key) {
+        function next() {
-            case 'ArrowRight':
-                e.preventDefault();
             var nextButton = document.querySelector('.nav-chapters.next');
             if (nextButton) {
                 window.location.href = nextButton.href;
             }
-                break;
+        }
-            case 'ArrowLeft':
+        function prev() {
-                e.preventDefault();
             var previousButton = document.querySelector('.nav-chapters.previous');
             if (previousButton) {
                 window.location.href = previousButton.href;
             }
+        }
+        switch (e.key) {
+            case 'ArrowRight':
+                e.preventDefault();
+                if (html.dir == 'rtl') {
+                    prev();
+                } else {
+                    next();
+                }
+                break;
+            case 'ArrowLeft':
+                e.preventDefault();
+                if (html.dir == 'rtl') {
+                    next();
+                } else {
+                    prev();
+                }
                 break;
         }
     });