Add operation for attaching an already existing dom element.

[Olian04]

API:

const canvas = document.createElement('canvas');
render(_=>_
   .attach(canvas)
);

Implementation: (in buildNode)

const ctx: VNode = {
    tag: tagType,
    value: null,
    text: '',
    events: {},
    props: {},
    children: [],
    external: null, // <-----
};
const builderCtx: BuilderCTX = {
    attach(elem) {
        const child = newVNode({
            tag: elem.nodeName.toLowerCase(),
            external: elem,
        });
        ctx.children.push(child);
        return this;
    }
}

[Olian04]

Might invert this. Changing it from attach to ref. Could help with SSR integration (#49).

let canvas;
render(_=>_
   .child('canvas', _=>_
      .ref({ canvas })
   )
);