feat: basic skeleton for Aurelia 2 plugin project

[3cp]

This is ready for early testing. Use:

npx makes aurelia/new#plugin-skeleton

Then either choose a default plugin or customise a plugin. The result project has some explanation in readme file on how to consume the plugin.

Known issue: If plugin project is compiled using non-shadow-dom setup, the style applied to the custom element inside the plugin would NOT work properly in app built with shadow-dom.

For example, in app's my-app.html template, you try to use an custom-element provided by a plugin

<!-- any element html template in the app creates a shadow-dom boundry -->
<!-- if the plugin element has global style (not ShadowDOM style), it would not
get applied to the following element inside a shadow-dom boundry -->
<some-plugin-element-with-style></some-plugin-element-with-style>

[3cp]

@bigopon @EisenbergEffect see the above issue for plugin's css style. Maybe we should recommend plugin authors to always use ShadowDOM mode for plugin? We certainly need to use shadow-dom as the default option for plugin project.

[bigopon]

Let's go with shadow DOM as default recommendation, with some heading that reminds folks this could cause friction, and let folks choose how to distribute/enforce the usage of their plugins.