`<html-example>` custom element

Create new custom element for live editing markup examples in our docs.

Should support:
- toggle rtl / ltr
- toggle writing mode
- toggle missing.css on/off
- buttons for selecting color ways
- toggle dark/light mode
- copy to clipboard
- allow/preserve omitted tags (pre-parsed HTML)

Possible inspo:
- https://overtype.dev/demo
- https://live.prismjs.com/

Brainstorm:
- Use `` or `<pre>`?
- make a custom element that copies/moves its contents to the shadow, then just toggle adoptedStylesheets to disable missing.css