App booted before DOM ready without jQuery

[chancancode]

ember.js/packages/@ember/application/lib/application.js

Lines 485 to 491 in b049c37

	waitForDOMReady() {
	if (!this.$ || this.$.isReady) {
	schedule('actions', this, 'domReady');
	} else {
	this.$().ready(bind(this, 'domReady'));
	}
	},

Without jQuery, this code no longer works and boots the app immediately (as soon as the bundle is loaded, but possibly before DOM read).

Workaround for now using an initializer:

export function initialize(app) {
  if (document.readyState === 'loading') {
    app.deferReadiness();

    let callback = () => {
      if (document.readyState !== 'loading') {
        app.advanceReadiness();
        document.removeEventListener('readystatechange', callback);
      }
    };

    document.addEventListener('readystatechange', callback);
  }
}

export default {
  initialize
};

We should reimplement $.ready() without relying on jQuery.

[kanavpruthi]

I am a beginner at contributing and would like to work on an issue here.