feat(css): add web component APIs
feat(css): add web component APIs

- adds [`:host`](https://developer.mozilla.org/en-US/docs/Web/CSS/:host()) and `:host()`
- adds [CSS Shadow Parts](https://www.w3.org/TR/css-shadow-parts-1/)
- adds [CSS Custom Properties](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_custom_properties)
kevinkirkup/vim-snippets,honza/vim-snippets,honza/vim-snippets,kevinkirkup/vim-snippets,craigp/vim-snippets,craigp/vim-snippets