Add support for Shadow DOM parents

[dermotduffy]

PR Checklist

Please review the guidelines for contributing to this repository.

• [x] I am requesting to pull a topic/feature/bugfix branch (right side). In other words, not main.
• [x] I have run yarn test against my changes and tests pass.
• [x] I have added tests to prove my fix is effective or my feature works. This can be done in the form of unit tests in test/unit/ or a new or altered demo in demo/.
• [x] I have added or edited necessary types and generated documentation (yarn docs), or no docs changes are needed.

Description

Add support for the parent element being a Shadow Root host. Right now, this library will support elements in Shadow DOM, but not if the parent element is the shadow root host (i.e. use of this library at the top-level of a web component).

Also includes very minor typing fixes/clarifications.

Fixes: #

[dermotduffy]

@timmywil Gentle ping -- any thoughts on this PR?