Consider using Constructed Stylesheets for web component styles
I was wondering if it might be worthwhile to consider using [constructed stylesheets](https://github.com/WICG/construct-stylesheets/blob/gh-pages/explainer.md) on the web component class.

`box-icons` currently adds styles to the shadow DOM with a `<style />` tag, and it looks like that's being used by ShadyCSS for polyfill purposes. The web component code was last touched 4+ years ago. With constructed stylesheets being supported by [~87% of browsers](https://caniuse.com/mdn-api_cssstylesheet_cssstylesheet) these days (with most of what's not compatible being versions of Safari from before this year), maybe we could knock down some of the code in the base class?

I'd be happy to jump in on this, but I wanted to make sure there isn't a good reason not to before I put the time in.