Require loaders

[fealebenpae]

Implement a pluggable loader infrastructure in our module loader, similar to RequireJS plugins.

[PanayotCankov]

To support the backward compatibility after "Publish NativeScript JavaScript modules as npm packages", we will need to alter the logic in the require to look in the @nativescript scope as a last resort. So that a require("geolocation") could continue to work along with require("@nativescript/geolocation").