Deprecated API harms Lighthouse score

Hi there, I just noticed that when I run Lighthouse with this polyfill, in Chromium browsers [these lines](https://github.com/webcomponents/template-shadowroot/blob/941aeecc24b156ed2b47d77f334d23e1f66922b7/src/_implementation/feature_detect.ts#L21C1-L23C8) are flagged as a deprecated API and severely reduce the Best Practices result. It seems from [here](https://chromestatus.com/feature/5116094370283520) that this needs to be updated to use `parseHTMLUnsafe()` instead? I can put in a PR for this if that's acceptable