Remove restriction on external module augmentations introducing new top-level names

[RyanCavanaugh]

Module augmentation cannot introduce new names in the top level scope

This error is stopping us from accurately representing UMD-augmenting modules like moment-range. While it's a correct restriction for ES6 values, it's not correct for legacy modules, nor is it a correct restriction for types (e.g. you might want to write an interface in a module augment to provide a type of a new overload parameter)

[mhegazy]

@vladima any concerns here?

[kitsonk]

Just ran across this while trying to convert to UMD definitions, glad it is fixed. Just wanted to mention too, there is nothing preventing an ES6 module from grabbing a reference to global and augmenting it. For example, this is completely valid:

'use strict';
const global = Function('return this')();
global.foo = 'bar';
console.log(foo); // logs "bar"