YAML Anchors and References Break Schema Validation and Suggestions

## Describe the bug
When user anchors or references in CloudFormation YAML files, schemas stops working when anchors/references are used.

Originally chased with `redhat.vscode-yaml` in https://github.com/redhat-developer/vscode-yaml/issues/425, but became clear the issue was being caused by this extension.

Since CloudFormation supports anchors and references through the `aws cloudformation package` path, would it make sense to fix the underlying issue?

## Expected Behavior
It works as normal.

## Current Behavior
Following error keeps repeating as elements are hovered.
```
Error thrown while requesting schema "Error: end of the stream or a document separator is expected" when calling the registered contributor "cloudformation"
```

## Steps to Reproduce

1. Add an anchor to any element
``` yaml
...
Tags: &tags
- Key: Application
Value: MyApp
...
```
## Environment
- [x] Windows
- [ ] Mac
- [ ] Linux
- [ ] other (please specify)

Start at the registered "cloudformation" schema contributor and reproduce the repeated schema error with the YAML anchor example. Compare the behavior with redhat.vscode-yaml issue 425, then verify that CloudFormation schema validation and suggestions work normally for anchors and references without the error recurring.

Written by the indexing model from the issue text.