Split ContextMenu actions

This should have no user-visible impact.

For vim `.` to repeat it's important that actions are replayable.
Currently editor::MoveDown *sometimes* moves the cursor down, and
*sometimes* selects the next completion.

For replay we need to be able to separate the two.

assets/keymaps/default.json

@@ -515,6 +515,17 @@
       "enter": "editor::ConfirmCodeAction"
     }
   },
+  {
+    "context": "Editor && (showing_code_actions || showing_completions)",
+    "bindings": {
+      "up": "editor::ContextMenuPrev",
+      "ctrl-p": "editor::ContextMenuPrev",
+      "down": "editor::ContextMenuNext",
+      "ctrl-n": "editor::ContextMenuNext",
+      "pageup": "editor::ContextMenuFirst",
+      "pagedown": "editor::ContextMenuLast"
+    }
+  },
   // Custom bindings
   {
     "bindings": {