🚀 [Feature]: Support anchors `&` and aliases `*`

[MariusStorhaug]

Anchors and aliases let a YAML document reference the same node multiple times.

Request

ConvertFrom-Yaml should resolve aliases to the anchored node:

defaults: &defaults
  timeout: 30
  retries: 3

production:
  <<: *defaults
  host: prod.example.com

Note: the << merge key is a separate (legacy YAML 1.1) feature and is not part of YAML 1.2.2 core. Decide whether to support it as an opt-in extension.

ConvertTo-Yaml does not need to emit anchors automatically (the spec allows but does not require it). Round-tripping anchors is out of scope for v1.

Spec examples covered

• Example 2.10 — Node "Sammy Sosa" appears twice
• Parts of Example 2.27 (Invoice)

Implementation notes

• Anchor scope is per-document. Multi-document support (separate issue) is a prerequisite for full correctness.
• Cyclic references should be detected and rejected (or surfaced as an error) since PowerShell objects do not naturally represent graphs.