feat(editor): Add keyboard shortcuts for completion navigation (#3382)

## 📝 Summary

Add Alt-J/K (Ctrl-J/K on macOS) shortcuts to navigate through the
completion menu, inspired by Vim:
- Alt-J/Ctrl-J: Move down in completion list
- Alt-K/Ctrl-K: Move up in completion list

This provides a more efficient way to navigate completions without using
arrow keys.


https://github.com/user-attachments/assets/3c261966-e2cb-454e-9862-d26380d32cdf


## 📋 Checklist

- [x] I have read the [contributor
guidelines](https://github.com/marimo-team/marimo/blob/main/CONTRIBUTING.md).
- [x] For large changes, or changes that affect the public API: this
change was discussed or approved through an issue, on
[Discord](https://marimo.io/discord?ref=pr), or the community
[discussions](https://github.com/marimo-team/marimo/discussions) (Please
provide a link if applicable).
- [ ] I have added tests for the changes made.
- [x] I have run the code and verified that it works as expected.

## 📜 Reviewers

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@akshayka OR @mscolnick

frontend/src/core/codemirror/cm.ts

@@ -4,6 +4,8 @@ import {
   closeBrackets,
   closeBracketsKeymap,
   startCompletion,
+  moveCompletionSelection,
+  completionStatus,
 } from "@codemirror/autocomplete";
 import {
   history,
@@ -170,6 +172,30 @@ export const basicBundle = (opts: CodeMirrorSetupOpts): Extension[] => {
         },
         preventDefault: true,
       },
+      {
+        key: "Alt-j",
+        mac: "Ctrl-j",
+        run: (cm) => {
+          if (completionStatus(cm.state) !== null) {
+            moveCompletionSelection(true)(cm);
+            return true;
+          }
+          return false;
+        },
+        preventDefault: true,
+      },
+      {
+        key: "Alt-k",
+        mac: "Ctrl-k",
+        run: (cm) => {
+          if (completionStatus(cm.state) !== null) {
+            moveCompletionSelection(false)(cm);
+            return true;
+          }
+          return false;
+        },
+        preventDefault: true,
+      },
     ]),
     keymap.of([...historyKeymap, indentWithTab]),
   ];