refactor Document, element, and related files

[jderochervlk]

This one is larger than the previous PRs, but it builds on the same ideas.

I don't love DOMTree, but I needed a spot to store types that are used across other modules without increasing the size of DOM. If I can get DOM down more I might remove this later.

Main changes:

• Adds new DOM structure modules to the build: DOMTree, HTML, and SVGElement.
• Moves many types out of the old broad DOM/DomTypes style into more specific modules:
  • Core tree types use DOMTree.node, DOMTree.element, etc.
  • HTML element types use concrete modules like HTMLImageElement.t, HTMLVideoElement.t, HTMLCanvasElement.t.
  • SVG image references move to SVGElement.svgImageElement.
  • Shared non-tree DOM types remain under DOM, like DOM.domRectReadOnly, DOM.canvasRenderingContext2D, DOM.videoFrame, DOM.fileList.
• Shrinks src/Base/DOM.res by extracting large chunks of element/node/document-related structure into dedicated files.
• Refactors APIs across Canvas, Window, IntersectionObserver, ResizeObserver, UIEvents, WebAudio, EncryptedMediaExtensions, XPath, and tests to stop depending on DomTypes.
• Moves some formerly separate *Types definitions directly into their owning modules, notably MutationObserver, and deletes MutationObserverTypes.res.
• Updates tests and unmonorepo rewrite expectations to point at the new module locations, for example Element.t instead of DomTypes.element.

[brnrdog]

I liked the types living within their interfaces, makes them much more discoverable than before with the DOMTypes. I think the DOMTree is fair given the recursive types.

I'm just missing a .t for Element/Node/HTMLElement, like we do for other modules. What if we add a small alias on each like:

type t = DOMTree.element
include Impl({type t = t})