Allow for parallel babel builds

[jrjohnson]

This addon causes the app to opt out of Babel parallel builds.

Reproduction

To reproduce:

1. Create a new app

npx ember-cli new test-app && cd test-app
// Remove ember-cli-template-lint as this also does not support parallel builds
npm uninstall ember-cli-template-lint
ember install ember-page-title

2. Add the following to your ember-cli-build.js to throw a hard error on your Ember build if parallel builds aren't possible

'use strict';

const EmberApp = require('ember-cli/lib/broccoli/ember-app');

module.exports = function(defaults) {
  let app = new EmberApp(defaults, {
    'ember-cli-babel': {
      throwUnlessParallelizable: true
    }
  });
  return app.toTree();
};

3. Run npm run build

It will generate the following error:

[broccoli-persistent-filter:Babel > [Babel: test-app]: Babel: test-app] was configured to `throwUnlessParallelizable` and was unable to parallelize a plugin. 
plugins:
1: name: unknown, location: unknown

Please see: https://github.com/babel/broccoli-babel-transpiler#parallel-transpilation for more details

Explanation

I stole this code and most of the text for this PR from @patocallaghan and the same change at ember-simple-set-helper which duplicates similar working patterns like ember-cp-validations

Alternative

The purpose of this babel transform seems to be converting from an old {{page-title}} helper syntax to the current {{title}} since several major version and almost 5 years have passed since this change was made this transform could be removed and a new breaking major version released.

[knownasilya]

@jrjohnson I'm all for changing the helper and releasing as a new major version. I will merge this since it's here now, but we can remove once we rename the helper.

[jrjohnson]

Upon further review the mod goes from {{title}} to {{page-title}} and page-title is the current helper name. So this seems like it should remain unless the right path is to update docs and recommend that {{page-title}} be used instead?

[knownasilya]

@jrjohnson I need to add a deprecation to {{title}}, then we can do a major release.