Add VSCode extension recommendations

To make it easier for new contributors, suggest extensions for the tools we use

frontend/.vscode/extensions.json

@@ -0,0 +1,7 @@
+{
+    "recommendations": [
+        "stylelint.vscode-stylelint",
+        "dbaeumer.vscode-eslint",
+        "esbenp.prettier-vscode"
+    ]
+}