feat: implement block scalar chomping handling for serialization and deserialization

[glucaci]

This pull request adds robust support for YAML block scalar "chomping" indicators (|, |-, |+) to both the reader and writer, ensuring correct handling of trailing newlines in multi-line strings. It also introduces logic to automatically use the literal block style when serializing multi-line strings and includes comprehensive tests to verify roundtripping and edge cases.

Block scalar chomping support:

• Added a private field _blockScalarChomping and logic to track and store the chomping indicator in both ParseLiteralBlockScalar and ParseFoldedBlockScalar methods in Utf8YamlReader.Parsing.cs and Utf8YamlReader.cs. [1] [2] [3]
• Updated ProcessBlockScalarContent to apply chomping rules: strip (-), keep (+), or clip (default), ensuring correct trimming or preservation of trailing newlines when reading block scalars. [1] [2] [3]
• Modified the Read() method to pass the chomping indicator to ProcessBlockScalarContent.

Writer improvements for multi-line strings:

• Updated WriteScalarValue in Utf8YamlWriter.cs to auto-detect multi-line strings and serialize them using literal block style (|, |-, or |+) based on the string's trailing newlines, with a dedicated WriteLiteralScalarValue helper. [1] [2]

Testing and validation:

• Added extensive tests in PrimitiveSerializationTests.cs to verify:
  • Serialization of multi-line strings uses literal block style with correct chomping indicators.
  • Correct round-tripping of multi-line strings with and without trailing newlines.
  • Accurate parsing of block scalars with strip chomping, even when followed by other YAML content.