fix: onError consistency, scalar shorthand tests, README docs for !reference/!reference-all shorthands

[Copilot]

Review feedback on the scalar shorthand feature for !reference and !reference-all identified three issues: error reporting inconsistency, missing test coverage, and stale README docs.

Changes

• Error handling (src/Reference.ts, src/ReferenceAll.ts): Replaced throw new Error(...) with onError(...) callback in both scalar shorthand resolvers, aligning with the pattern used in all other tag resolvers (e.g., illegalReferenceOnSequence, illegalFlattenOnScalar).

• Tests (test/parser.test.ts): Added 4 tests covering scalar shorthand for both tags:

  • Valid string path/glob resolves to correct Reference/ReferenceAll instance
  • Numeric literal is coerced to string by the YAML parser before reaching the resolver (documents the behavior — !reference 123 yields path: "123")

  # These now have test coverage:
  database: !reference "./config/database.yaml"
  configs:  !reference-all "./refs/*.yaml"

• README (README.md): Added scalar shorthand syntax examples, updated the "Note" to reflect both mapping and shorthand forms are valid, and removed the outdated "Deterministic Behavior" bullet stating scalar syntax is rejected.

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