Support token for short syntax without value property

## Proposal

At now all tokens should have property "value" for be valid. Suggests short syntax without "value", example:

**full syntax**

```yaml
token:
value: token-value
```

**short syntax**

```yaml
token: token-value
```

Start by tracing how YAML tokens are parsed and validated, since the issue names no file or test entry point. Compare the existing value-property validation with the proposed scalar short syntax. Done means accepting `token: token-value` while preserving the existing nested form and adding coverage for both forms.

Written by the indexing model from the issue text.