feat: Add StreamingMarkdownDecoder for LLM streaming scenarios

[Feiyu-Liu]

Summary

Adds StreamingMarkdownDecoder for efficient incremental parsing of LLM streaming output.

Key Features

• Line-level state tracking to avoid re-parsing closed blocks
• Shared parsing logic between MarkdownDecoder and StreamingMarkdownDecoder
• 2-4x performance improvement for streaming scenarios

Changes

• lib/src/parser.dart: Add StreamingMarkdownDecoder and refactor shared parsing logic
• benchmark/streaming_performance_benchmark.dart: Add performance benchmark

Benchmark Results

• Small content (50 lines): 2.84x faster
• Medium content (200 lines): 1.97x faster
• Large content (500 lines): 3.67x faster

Testing

All 80 tests pass including 18 new streaming-specific tests.

[mike-doctorina]

@Feiyu-Liu because this is package, update, please, version at pubspec.yaml and changelog.md as well.
And fix formatting at lib/src/parser.dart as well