refactor: rewrite Module as an ES6 class (#4151)

This PR rewrites `module.js` to use a native ES6 `class` instead of
`Class.extend()` - the same old inheritance helper that was removed from
`node_helper.js` in #4147.

The normal module API stays the same: modules still use
`Module.register({...})`. Internally, `Module.create()` now creates a
named subclass for each module, copies over a cloned definition, and
only calls `init()` when it is actually a function.

Outcome: one less file in the browser bundle, no more magic `_super()`
wiring, better stack traces, and tests for the module creation path that
did not exist before. `module.js` is also now a plain class with no
external dependencies - an intentional step towards `export default
Module` when browser scripts eventually move to native ES modules.

Since these changes touch the core of how modules are created, I'd
appreciate a close review and any feedback on edge cases I may have
missed.

defaultmodules/newsfeed/newsfeed.js

@@ -148,7 +148,7 @@ Module.register("newsfeed", {
 			}
 			return Promise.resolve(wrapper);
 		}
-		return this._super();
+		return Module.prototype.getDom.call(this);
 	},
 
 	//Override fetching of template name