[Feature] Support `anchors` top-level key for yaml anchors

### Is this your first time submitting a feature request?

- [x] I have read the [expectations for open source contributors](https://docs.getdbt.com/docs/contributing/oss-expectations)
- [x] I have searched the existing issues, and I could not find an existing issue for this feature
- [x] I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

### Describe the feature

Context:
- https://github.com/dbt-labs/dbt-core/issues/13197
- https://github.com/dbt-labs/docs.getdbt.com/issues/7723

dbt Core should also support the new `anchors` as well, so that:
- projects which migrate to the new syntax remain back-compatible with dbt Core
- users can adopt the new jsonschemas for more rigorous real-time yaml validation

### Describe alternatives you've considered

dbt Core does not support the new `anchors` syntax for yaml anchors, leading to inconsistencies / hard cutovers during Core → Fusion migrations

### Who will this benefit?

- All users of yaml anchors in dbt Core (stronger validation!)
- Users migrating projects to Fusion

### Are you interested in contributing this feature?

_No response_

### Anything else?

_No response_