Add ruler at column 120 for VS Code

This provides some visual guidance, e.g. for comments not touched
by rustfmt, and corresponds to the value in rustfmt.toml.

Author: skoe

.vscode/settings.json

@@ -12,4 +12,7 @@
   "[cpp]": {
     "editor.formatOnSave": false
   },
+  "editor.rulers": [
+    120
+  ],
 }