[ember-cli] Ability to hook into build process without addons

[mehulkar]

I should be able to hook into the Ember build process without needing an addon. Something like this:

new EmberApp({
  treeFor() {
  }
})

Not totally sure how this would work or the semantics, but I would like a lightweight way to affect the broccoli tree. I suspect that somewhere in the process of calling the hooks for all addons, these hooks from the EmberApp definition could also be called pretty easily.

• https://github.com/ember-cli/ember-cli/blob/v3.12.0/lib/broccoli/ember-app.js#L1665-L1682
• https://github.com/ember-cli/ember-cli/blob/v3.12.0/lib/broccoli/ember-app.js#L734-L744

There was some discussion in Discord #ember-cli about in-repo addons being used for this case, but IMO in-repo addons do more harm than good:

1. They give developers the illusion that lib code is not owned by the project / shouldn't be touched.
2. It's not entirely clear how to configure tests of lib code to run in the normal test suite
3. They make it easy to go half-way to creating good abstractions that can actually be shared

Secondly, providing these hooks would allow developers to gradually build good build time abstractions: (1) inline in build file (2) in-repo addon (3) external addon.

[mehulkar]

Opened #578 for this.