feat: add async APIs for ToonEncoder and ToonDecoder

[parsapanahpoor]

Summary

Adds asynchronous API support to the TOON library, enabling non-blocking encoding and decoding operations.

Changes

ToonEncoder

• EncodeAsync<T>() - Async encoding with default/custom options
• EncodeToBytesAsync<T>() - Async encoding to UTF-8 bytes
• EncodeToStreamAsync<T>() - Async encoding directly to stream

ToonDecoder

• DecodeAsync() / DecodeAsync<T>() - Async decoding from string
• DecodeAsync(stream) / DecodeAsync<T>(stream) - Async decoding from stream

All methods support CancellationToken for cancellation.

Why?

• Modern .NET applications rely heavily on async/await patterns
• Stream-based operations benefit from true async I/O
• Follows .NET library design guidelines

Testing

Added 15 new tests in ToonAsyncTests.cs covering:

• Basic encode/decode operations
• Stream operations
• Cancellation token support
• Round-trip verification
• Error handling

Checklist

• Code follows project conventions
• XML documentation added
• Unit tests added and passing
• No breaking changes

[parsapanahpoor]

Hi maintainers! 👋

This is my first contribution to this project. I've added async API support for both ToonEncoder and ToonDecoder.

Key additions:

• EncodeAsync<T>() and variants
• DecodeAsync() and DecodeAsync<T>() with stream support
• Full CancellationToken support
• 15 new unit tests (all passing)

I'd appreciate any feedback or review when you have time. Thanks! 🙏

[239573049]

Approve