Add DOM - core, events, ranges, tree walker etc

[autonome]

See #1380 for discussion on DOM as a standalone feature, ranges, etc:

• we seem to be sticking w/ putting things w/ where they're used, so putting ranges w/ selection api
• there was no conclusion on a general "DOM" feature

Looking at the spec-generated DOM draft, some thoughts and questions:

• events seem like something that should stand alone as one or more features, and Caniuse has various separate related features: addEventListener, passive: true, CustomEvent, EventTarget.dispatchEvent and more
• Document, DocumentFragment and DocumentType are proposed features in Add document, document fragment, document type #1983
• MutationObserver seems standalone and Caniuse treats it that way too
• the Node and Element keys together have so many "use it 10,000 times per day" APIs, could be a standalone feature probably
• qS/A are standalone on Caniuse, which seems right
• there's a set of list-ish APIs HTMLCollection, NamedNodeMap, NodeList, NodeIterator` - if we go big-dom-feature, could leave them all in there, for now at least
• and then some bits like Attr, Comment, CharacterData, Text etc, some of which will align w/ their HTML spec counterparts - these each seem like standalone features of the web

Aside from the list-ish bits, there's not much left for a standalone DOM feature if we do the above... and those might actually be ok living in Document.

[ddbeck]

I'm feeling pretty positively toward this so far. I'm inclined toward lumping more stuff into the DOM feature itself (Node, Element, and maybe querySelector), which I've noted in a few places. But this is a good outline of what to do with a LOT of keys.

[autonome]

Implemented the DOM rollup approach, and review feedback integrated.

Edit: Er, and subsequent files updated also.