Formalize App booting

[kategengler]

ember-cli pushes code onto bundle that causes app to boot but there should be public api to boot app. If well-designed, mounting an engine should be the same API. Where in dom does it render, handle to URL state (maybe subpart), inherited services, etc.

Would want to retrofit fastboot and testing to use the new API.

Composes with stuff in the es modules RFC.

Simple v2 addons shouldn't need a full npm package/monorepo just to test.

• Consider Engines

Needs to consider routing changes for the URL handing APIs by making it generic enough.

Blocked on design questions around services.

[kategengler]

May not be a requirement of Polaris