Fork the postfix cookbook to work around a bug

I ran into the issue described in
https://github.com/sous-chefs/postfix/issues/148
and couldn't figure out a way to work around it without forking it.

cookbooks/postfix/.vscode/extensions.json

@@ -0,0 +1,8 @@
+{
+  "recommendations": [
+    "chef-software.chef",
+    "Shopify.ruby-lsp",
+    "editorconfig.editorconfig",
+    "DavidAnson.vscode-markdownlint"
+  ]
+}