Cordova events singleton object

[brodycj]

• Refactor events object into a real ES6 class (and replace var with const & let)
• Use a real singleton object as described in https://derickbailey.com/2016/03/09/creating-a-true-singleton-in-node-js-with-es6-symbols/, in a similar fashion as done for CordovaLogger in PR Refactor CordovaLogger to singleton class #53

Using a real singleton object means that we would no longer have to follow the cordova-common singleton rule when publishing tools releases.

We may need to increase the minor version number if this change is included in cordova-common@3.

I would love to get this into the Cordova 9 release as discussed in apache/cordova#10, if possible.

[dpogue]

Does EVENTS_RECEIVER also need to make sure there is only ever one instance?

[brodycj]

Whichever CordovaEventEmitter instance is used should always use the same EVENTS_RECEIVER variable, from the same events.js module.