Update for Ember 4 and Embroider

[ef4]

This is rewrite into modern Ember patterns, and repackaging as a v2 formatted addon. There are semver major changes:

• minimum supported ember goes up to 3.25
• minimum supported node goes to 12
• apps must have ember-auto-import 2
• moment or moment-shim have become optional peerDependencies. They're optional if you only use the new {{pikaday}} modifier. One or the other must be present if you use the backward-compatible <PikadayInput /> or <PikadayInputless/> components.
• apps that want Pikaday's default CSS (you probably do) must include them explicitly. This replaces the excludePikadayAssets option. See below.

The <PikadayInput> and <PikadayInputless/> components should have no changes (and all their tests are passing unchanged).

There is also now a modifier to directly invoke Pikaday on an element:

<input {{pikaday value=this.startDate onSelect=this.setStartDate}} />

CSS Handling

None of these components hard-code their CSS, because this addon historically has an option to leave the CSS out. Instead, consuming apps that want the standard CSS should pull it in. For example:

// app/modifiers/pikaday.js
import "ember-pikaday/pikaday.css";
export { default } from "ember-pikaday/modifiers/pikaday";

[ef4]

This PR includes a GitHub Actions setup, you can see it running here: https://github.com/ef4/ember-pikaday/pull/1