add "part" attribute to treescope_root

[mhuebert]

Adding a "part" attribute to the root element would allow users of Treescope to target content in the shadow DOM with CSS rules, eg.

treescope-container::part(treescope_root) {
  font-family: Inconsolata;
}

(context: we use Treescope in our documentation website, and would like consistent style/formatting)

[danieldjohnson]

Thanks!

The stability of the treescope DOM structure isn't guaranteed across releases, so there's a chance this may break eventually, but it seems like a reasonable solution for now at least.