Switch from an initializer to manual extendResolver usage

[mkszepp]

ember-resolver v13 is now a native class instead of a EmberObject.

With EmberObject there was possible to reopen the class for making changes to pluralizedTypes.
This isn't anymore working with native classes. For this reason we need to provide users a short instructions like they can configure ember-can inside there app.

fix #185, close #184
implement logic from comment #184 (comment)

The app.js / .ts looks at end like:

import Application from '@ember/application';
import Resolver from 'ember-resolver';
import loadInitializers from 'ember-load-initializers';
import config from 'my-app/config/environment';
import { extendResolver } from 'ember-can';

export default class App extends Application {
  modulePrefix = config.modulePrefix;
  podModulePrefix = config.podModulePrefix;
  Resolver = extendResolver(Resolver);
}

loadInitializers(App, config.modulePrefix);

Why import { extendResolver } from 'ember-can'; instead of import Resolver from 'ember-can';?
The advantage is, that when an app has already extended the Resolver he can pass his extended resolver and the extending order is not relevant.

PR changes:

• Add function for app
• Add tests for ember-resolver < 12 with current latest ember-source LTS
• Add instructions for users in install documentation

[wozny1989]

Nice @mkszepp, looks very nice! 👍

[RobbieTheWagner]

This is awesome. Great work @mkszepp, thank you!