Fix #809 incorrect ViewNode prev and next siblings after appendChild/insertBefore

[delaneyb]

Fixes #809

[rigor789]

Nice find @delaneyb! Could you add a test similar to this

nativescript-vue/__tests__/renderer/ViewNode.test.js

Lines 106 to 121 in b46498c

	test('insertBefore sets the correct node relations', () => {
	let node = new ViewNode()
	let childNode = new ViewNode()
	let refNode = new ViewNode()
	node.childNodes = [refNode]
	refNode.parentNode = node
	node.insertBefore(childNode, refNode)
	expect(node.childNodes.length).toBe(2)
	expect(childNode.parentNode).toEqual(node)
	expect(childNode.prevSibling).toBeFalsy()
	expect(childNode.nextSibling).toEqual(refNode)
	expect(refNode.prevSibling).toEqual(childNode)
	expect(refNode.nextSibling).toBeFalsy()
	})

to assert the childNode.prevSibling.nextSibling is set? I can do it a bit later as well, so no problem if you can't - will try to get to merging/releasing a patch sometime this week.

[rigor789]

Awesome, thanks!