Change import statement to use named import for App

[NullVoxPopuli]

This import must match this export:

ember-addon-blueprint/files/demo-app/app.gts

Line 10 in 2a95a64

export class App extends EmberApp {

Issue introduced in
#89

@evoactivity mentioned a thing that is probably worth mentioning reasoning behind:

A named import diverges from the main app Blueprint:

but only because the main app blueprint is beholden to previous blueprints.
I argue that:

1. an import in your app from your app should be a named import. (Otherwise why name anything at all? Named exports also help a toooon with import suggestions)
2. Leave export default for 'the framework'.

This is also base common expectations for the ecosystem for like.. 10 years, so there is a lot of precedent.

[NullVoxPopuli]

Merged because #89 was released in 0.14 earlier today