Use extends option

[dy]

There's a use-case for custom elements, that allows extending built-in components https://developer.mozilla.org/en-US/docs/Web/Web_Components/Using_custom_elements#Customized_built-in_elements. That allows to solve inherent problems of JSX, as stated in the preact issue developit/htm#81

If remount would take extends: tagName as an option, it would enable component-less JSX:

htm`
<nav is="app-nav"/>
<main is="app-page">
  <header slot="header" />
  <section slot="content">...</section>
  <footer slot="footer" />
</main>
`