feat: support .NET 9 and .NET 10

[ling921]

Summary

• target the ToastUIEditor library and samples at .NET 9 and .NET 10
• add enum and enum-dictionary JSON deserialization and improve JS callback/resource disposal
• add unit tests for options, validation, language handling, and widget rules
• add GitHub Actions for build/test and automatic NuGet publication on releases or manual dispatch
• update README and package version to 1.1.0

Validation

• dotnet build ToastUIEditor.sln --configuration Release --no-restore
• dotnet test tests/ToastUIEditor.Tests/ToastUIEditor.Tests.csproj --configuration Release
• dotnet pack src/ToastUIEditor/ToastUIEditor.csproj --configuration Release --no-restore