Singleton Pattern Externs Inference Support

The Problem

Currently externs inference is well suited for interop patterns that involve foreign JS instances, for example a function that takes a instance of React.Element. However, the feature is poorly suited for the popular singleton pattern employed by libraries like jQuery and D3 where all the interesting operations are provided through a single instance. In this case, the user will see many warnings about generated externs on Object but this is exactly what the user wants.

Proposed Solution

Support additional metadata to support the singleton pattern and suppress undesirable warnings.

(def ^:singleton $ (js/require "jQuery"))

Alternative name - :js/singleton.