To enable us to take advantage of the UI tooling that we built for the
sidebar app in the "annotator" part of the client (eg. components,
linting, testing infrastructure), we're going to convert UI components
in the "annotator" code over to Preact as well. As a first step, this
commit converts the "adder" toolbar that presents the "Annotate" and
"Highlight" buttons.

 - Extract the logic for creating a shadow DOM root for the adder into
   a separate module (`annotator/util/shadow-root.js`) so we can re-use it for other UI
   elements in future and remove Shadow DOM v0 support since Shadow DOM v1 has
   been available for a long time now.

 - Replace the `adder.html` template file, manual element manipulation
   and manual event listener setup with an `AdderToolbar` Preact
   component (`annotator/components/adder-toolbar.js`), which uses the same
   tooling as components in the sidebar app

In this commit the tests for all the pieces of the adder are covered by
`adder-test.js`. Future commits will split out tests for the individual
pieces.