feat: provide getPageTitle test helper to consuming apps

[srsgores]

This PR allows consuming apps to assert a page's title (whether integration or acceptance test).

Usage documented in the README.md:

import { getPageTitle } from 'ember-page-title/test-support';

module('Acceptance | Register Page', function(hooks) {
  setupApplicationTest(hooks);

  test('visiting /register', async function(assert) {
    const registerURL = '/register';
    await visit(registerURL);

    assert.equal(currentURL(), registerURL);
    assert.equal(getPageTitle(), 'Register | Some Website');
  });
});

[knownasilya]

Some doc updates, otherwise looks good.

[srsgores]

Should be updated

[knownasilya]

@raido thoughts on this addition? Since the helper was internal only before.

[srsgores]

I'll clarify that part of our enterprise front-end is waiting on this PR to get merged. We had previously run our acceptance tests via a private helper as well, but this quickly became cumbersome when we needed to assert page titles on addon acceptance tests.

[srsgores]

@knownasilya, can we also bump package.json and release on npm?

[knownasilya]

Done

[raido]

I am a bit late here, but since this "private" helper is re-exported via index.js, it's fine. We can easily re-work internals.

We probably should add comment block so IDEs could give hints of expected parameters OR provide TypeScript interface file. I think proper JSDoc for starters would be nice.

This helper does have some internal logic comments in it, we should probably remove it or separate it still for testing specifically this addon vs public helper.

The Array.pop() workaround for consumers is not needed, it's only applicable for tests in this repository.