HTML rendering.

Implements web components on top of the component registry.

State lives at `::state` in the db, and has:
  ::class->fields-index
  ;; Mapping of `all prop name variations -> writable JS property name`
  ;; for element classes.

  ::css-link-elements
  ;; A set of all CSS `<link>` elements managed by this plugin.  Used
  ;; for quick access when we need to override the hrefs during hot
  ;; reload.  Only populated when hot reload is enabled.

  ::css-stylesheet-objects
  ;; A map of `normalized URL -> constructed style sheet`, so we can
  ;; reuse them.  Entries never expire.

  ::href-overrides
  ;; When shadow-cljs (or figwheel I presume) detects an FS change of
  ;; some stylesheet linked at the top level of the document, it'll force
  ;; the browser to reload it be appending some arbitrary 'version'
  ;; to the href's query string.  The plugin watches for those changes, and:
  ;;   1. Keeps track of the new HREF, so it can be used in newly created links
  ;;      and constructed stylesheets.
  ;;   2. Replaces the HREF in all matching `::css-link-elements`.
  ;;   3. Reloads all matching `::css-stylehseet-objects` with the new URL.
  ;;
  ;; Only populated when hot reload is enabled.