PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/nativescript-angular/blob/master/DevelopmentWorkflow.md#running-the-tests
• Tests for the changes are included.

What is the current behavior?

Ordering can be erratic when nesting ng-containers and templates. This is due to previousSibling not being implemented and comment nodes not being properly respected

<ng-container>
  <Label text="1" *ngIf="show"></Label>
  <Label text="2" *ngIf="show"></Label>
</ng-container>

will render:

2
1

What is the new behavior?

The node list is now doubly linked (previousSibling and nextSibling), allowing for faster processing of previousSibling and respecting the positioning of commentnodes.

<ng-container>
  <Label text="1" *ngIf="show"></Label>
  <Label text="2" *ngIf="show"></Label>
</ng-container>

will render
1
2

Fixes/Implements/Closes #[Issue Number].