Add support for web component features (custom elements and shadow DOM)

[calebdwilliams]

domino needs support for web component features like <template>, shadow DOM and custom elements. There is some prior art in this category as @skate/ssr and jsdom support these features. As domino is heavily used in Angular Universal and Angular has options to use native shadow DOM, this should be a supported feature.

[calebdwilliams]

@fgnass, is this something you would be interested in supporting or accepting a PR on? I'd be happy to work on it if I had a bit of a primer on how domino works.

[cscott]

domino supports the <template> element. Shadow DOM support would need implementing.