Add page up/down bindings to the Markdown preview (#33403)

First time contributor here. 😊

I settled on markdown::MovePageUp and markdown::MovePageDown to match
the names the editor uses for the same functionality.

Closes #30246

Release Notes:

- Support PgUp/PgDown in Markdown previews

assets/keymaps/default-linux.json

@@ -1068,6 +1068,13 @@
       "ctrl-shift-tab": "pane::ActivatePreviousItem"
     }
   },
+  {
+    "context": "MarkdownPreview",
+    "bindings": {
+      "pageup": "markdown::MovePageUp",
+      "pagedown": "markdown::MovePageDown"
+    }
+  },
   {
     "context": "KeymapEditor",
     "use_key_equivalents": true,

assets/keymaps/default-macos.json

@@ -1168,6 +1168,13 @@
       "ctrl-shift-tab": "pane::ActivatePreviousItem"
     }
   },
+  {
+    "context": "MarkdownPreview",
+    "bindings": {
+      "pageup": "markdown::MovePageUp",
+      "pagedown": "markdown::MovePageDown"
+    }
+  },
   {
     "context": "KeymapEditor",
     "use_key_equivalents": true,