Cn prototype

[bachbui]

This is a PR showing adding two custom plugins: a parabox container and a custom blockquote. The custom blockquote has the data model

<block-quote>
  <block-quote-body>...</block-quote-body>
  <block-quote-credit>...</block-quote-credit>
</block-quote>

and is rendered via custom HTML (ie a web component).
The plugin includes a button to insert a new blockquote and a floating toolbar to adjust its position as inset-left, full-width, or inset-right.

The custom parabox has the data model

<para-box>

and includes an insert button.

This PR also updates the sample text to include these new elements illustrate the upcast converters, and adds the developer tools to the build.