Reconsider the demo template syntax

[bennypowers]

api-demo's template syntax requires a special HTML syntax.

At the moment, Custom elements manifest and associated tooling has achieved a measure of maturity which it did not have when api-demo was originally factored.

The purpose of this issue to discuss the possibility of deriving demo knobs from plain HTML. The goal is that users would be able to write demo markup which exactly corresponds to final markup, without special template tags.

We would like to re-adopt api-viewer for patternfly elements and red hat design system, but we require the demo source files to be plain HTML, including imports and global css. example demo file