`<html-example>` custom element

[geoffrey-eisenbarth]

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 <textarea> or <pre>?
• make a custom element that copies/moves its contents to the shadow, then just toggle adoptedStylesheets to disable missing.css