Provide lib for modern DOM

This is basically a re-opening of #2910, which @mhegazy asked me to do. Nowadays TS has the `lib` compiler option that allow users to selectively choose which core definitions to include or not based on their target. It would be nice to add a new option similar to `dom` and `dom.iterable` for users targetting a modern DOM runtime. Not sure how to name this since W3C has moved from releases to a "living standard" model. For example, the living standard includes interfaces `ChildNode` with methods `after`, `before`, `remove` and `replaceWith`; `ParentNode` with methods `prepend` and `append`. Those methods have been in Firefox and Chrome for a long while, they're in preview in Edge (17035+). They are very convenient and there's no reason a dev that targets modern browsers shouldn't be able to do `"lib": "dom.living"` and use them. Unfortunately I am not sure if there's a list somewhere of all those new features... except going through the whole standard :(