Support dotnet build

[chenrui333]

Trying to do dotnet build with dotnet 7, but ran into some build issue:

MSBuild version 17.4.0+18d5aef85 for .NET
  Determining projects to restore...
  Nothing to do. None of the projects specified contain packages to restore.
/opt/homebrew/Cellar/dotnet/7.0.100/libexec/sdk/7.0.100/Microsoft.Common.CurrentVersion.targets(1229,5): error MSB3644: The reference assemblies for .NETFramework,Version=v3.5 were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks [/private/tmp/naturaldocs-20230221-8692-19lwp7j/NaturalDocs-Version_2.2/CLI/CLI.csproj]

relates to dotnet/msbuild#1333