Add Vitest extension to recommended VSCode extensions

Author: sjdemartini

.vscode/extensions.json

@@ -3,6 +3,7 @@
     "dbaeumer.vscode-eslint",
     "esbenp.prettier-vscode",
     "streetsidesoftware.code-spell-checker",
-    "yzhang.markdown-all-in-one"
+    "yzhang.markdown-all-in-one",
+    "vitest.explorer"
   ]
 }