[yaml_edit] Add configurable alias handling semantics via AliasBehavior

[sigurdm]

Stacked on top of #2587.

This PR introduces configurable YAML alias handling semantics to package:yaml_edit, allowing developers to opt into reference redirection or lazy Copy-On-Write (COW) materialization instead of throwing exceptions on alias mutations.

Motivation

Previously, any mutation targeting an alias reference (*anchor) or anchor definition (&anchor) in YamlEditor threw an AliasException. This prevented automated YAML editing in real-world files (such as CI/CD workflows, Docker Compose, and Kubernetes manifests) where YAML anchors and aliases are widely used for shared configuration templates.

Changes

• AliasBehavior Enum: Added an AliasBehavior enum with three options:
  • disallow (default): Any mutation touching an alias or anchor throws an AliasException (100% backwards-compatible).
  • reference: Mutating a property through an alias reference redirects the mutation to the underlying anchor definition, updating the template and automatically propagating the change to all references.
  • copyOnWrite (Lazy COW): Mutating a property through an alias reference unfolds the referenced collection inline at the reference location, decoupling it from the anchor template without modifying the original definition.
    • Lazy / Shallow Unfolding: Unfolds only the top-level container, preserving nested extra-template alias references (*nested) until those subtrees are directly modified.
    • Intra-Template Sub-Anchor Inlining: Strips intra-template sub-anchor definition tags (&sub_anchor) and expands intra-template aliases (*sub_anchor) inline so decoupled copies are completely self-contained and free from dangling references upon anchor deletion.
    • Mutating an anchor definition directly updates the template in place so inheriting references see the change.
• Anchor Tag Preservation: Added getAnchorTag to extract clean &anchorName tags from AST span text so that updating or replacing an anchor definition leaf preserves its anchor tag.
• Anchor Definition Detection: Added isAnchorDefinition to inspect whether a child node in a map or list is an anchor definition.
• Identity-based AST Tracking: AST alias detection and anchor resolution use object identity maps and sets (Set.identity(), Map.identity()) to ensure AST node tracking respects package:yaml object reuse.
• Block Map Header Preservation: Distinguishes block map headers (anchors and type tags) from keys during entry removal, preventing map-level tags/anchors from being wiped when the first entry is deleted.
• Alias Reference Pattern: Added aliasReferencePattern using positive lookahead (?=[\s,\[\]{}]|$) to match YAML delimiter boundaries, supporting anchor names with special characters (., /, @, +).
• Unit Tests: Comprehensive unit tests in test/alias_behavior_test.dart covering reference, copyOnWrite, disallow, intra-template sub-anchors, anchor name character sets, collection headers, and list appends.
• Example: Added example/alias_behavior_example.dart demonstrating usage.
• Changelog & Version: Bumped version to 2.3.0-wip in pubspec.yaml and documented changes in CHANGELOG.md.

[gemini-code-assist]

Code Review

This pull request introduces configurable alias and anchor mutation behaviors (AliasBehavior) to package:yaml_edit, allowing developers to choose between disallowing, referencing, or copying-on-write when modifying aliased YAML nodes. It also includes out-of-scope changes to package:pub_semver to fix parsing of pre-release and build identifiers. Feedback highlights several critical issues: getAnchorTag can return trailing whitespace leading to invalid YAML formatting in map mutations, _AliasEntryKey violates the == and hashCode contract, and potential NullThrownErrors exist when handling YamlNode keys. Additionally, the public getAnchorTag method lacks required documentation, and the out-of-scope pub_semver changes should be moved to a separate pull request.