(maint) Add visual ruler for line length

Previously it was a little difficult to know when your editing would exceed
the 120 line length limit.  This commit modifies the project settings to add
a simple visual ruler at the 120 char limit.

[skip ci]

Author: glennsarti

.vscode/settings.json

@@ -12,5 +12,8 @@
     },
 
     // Lock the TypeScript SDK path to the version we use
-    "typescript.tsdk": "./node_modules/typescript/lib"
-}
+    "typescript.tsdk": "./node_modules/typescript/lib",
+
+    // Add a visual ruler for the typescript linting line length
+    "editor.rulers": [120]
+}