Add lint rule to prevent ObservableScope resource leaks

The rule of thumb to avoid resource leaks is that you should never call ObservableScope methods in a callback unless the ObservableScope is directly passed to or created inside that callback. I had a go at codifying this as a lint rule.

eslint/index.js

@@ -0,0 +1,5 @@
+module.exports = {
+  rules: {
+    "no-observablescope-leak": require("./NoObservableScopeLeak").default,
+  },
+};