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/package.json

@@ -0,0 +1,4 @@
+{
+  "name": "eslint-plugin-element-call",
+  "version": "0.0.0"
+}