C# 9.0 support

[zorgesho]

Changes made in this pull request

• LangVersion is changed to 9.0
• IsExternalInit class is added to fix compiler errors when using records and init property modifier

[toebeann]

If you'd rather not use a package for the IsExternalInit polyfill, it should live in its own class file somewhere rather than being hidden in an unrelated class definition file.

However, this NuGet package takes care of that for you. If you use the new (since VS2017) PackageReference syntax in the .csproj to add the NuGet package, the file doesn't get added directly to your project (which also means it doesn't get accidentally deleted/modified by someone), it only gets compiled into the code if the target runtime doesn't include IsExternalInit already.

The only change required is to add the following to the .csproj (assuming you have NuGet set up to use PackageReference format):

<ItemGroup>
  <PackageReference Include="IsExternalInit" Version="1.0.0" PrivateAssets="all" />
</ItemGroup>

But if you'd really rather not use a package for this, I'd be much happier if the polyfill was moved into its own file.