Add vitest to recommended VSCode extensions (streamlit#9843)

## Describe your changes

Adds the official
[vitest-explorer](https://github.com/vitest-dev/vscode) to the list of
recommended VSCode extensions.

---

**Contribution License Agreement**

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project are made under the Apache 2.0 license.

.vscode/extensions.json

@@ -6,6 +6,7 @@
     "ms-python.python",
     "ms-python.debugpy",
     "charliermarsh.ruff",
-    "EditorConfig.EditorConfig"
+    "EditorConfig.EditorConfig",
+    "vitest.explorer"
   ]
 }