SmallSharp generated targets are not imported from dotnet build/run

[kzu]

See dotnet/msbuild#9512
The officially supported way of achieving what would be needed for VS+CLI builds is:

• Change emitted targets output path to $(MSBuildProjectExtensionsPath)$(MSBuildProjectFile).smallsharp.targets
• Document that users will need to set ImportProjectExtensionProps/ImportProjectExtensionTargets to true
• Warn (error?) if the above properties aren't set.