Re-architect plugin loading and asset detection logic (add Embroider support)

[thoov]

For apps (ember-cli-build.js):

let app = new EmberApp(defaults, {
  babel: {
    plugins: [...require('ember-cli-code-coverage').buildBabelPlugin()],
  },
});

For embroider apps (ember-cli-build.js):

let app = new EmberApp(defaults, {
  babel: {
    plugins: [...require('ember-cli-code-coverage').buildBabelPlugin({ embroider: true })],
  },
});

For in-repo addons (index.js):

module.exports = {
  name: require('./package').name,

  options: {
    babel: {
      plugins: [...require('ember-cli-code-coverage').buildBabelPlugin()],
    },
  },
};

For in-repo engines (index.js):

module.exports = EngineAddon.extend({
  // ...
  included() {
    this._super.included.apply(this, arguments);
    this.options.babel.plugins.push(...require('ember-cli-code-coverage').buildBabelPlugin());
  },
});

tests/index.html:

<script src="/coverage.js" integrity="" data-embroider-ignore></script>

tests/test-helpers.js:

import { forceModulesToBeLoaded } from 'ember-cli-code-coverage/test-support';

forceModulesToBeLoaded();

[thoov]

cc: @rwjblue

[rwjblue]

This is great!!! I left some comments inline, and I also think the README needs some updates; but overall I love it!

[rwjblue]

Some small inline comments, but overall this is looking really great!

I think other than the inline comments to address, we need to tweak the README on how to consume (I think your PR description code snippets are good enough there though).

[rwjblue]

I'd like to get this re-exported from the top level of @embroider/compat. Maybe we can follow up with that?

[thoov]

cc: @rwjblue Can you take another look at this

[villander]

@thoov @rwjblue and how about the standalone engines here?