Add .NET Core 3.0 target to Caliburn.Micro.Platform

[mjuen]

As #609 seems to be stuck at the moment I took the liberty to replicate the changes and include the review suggestions from @nigel-sampson.

I also renamed the folder containing the shared files from net46 to net46-netcore.

[nigel-sampson]

Looking really good, thanks. Need to sort out why PR builds aren't happening to test this out.

[nigel-sampson]

I've enabled PR builds on forks so any further changes should be built for you (in theory).

[mjuen]

I've enabled PR builds on forks so any further changes should be built for you (in theory).

Could you change the SDK to one that supports .NET Core 3.0?

Build is currently failing with The current .NET SDK does not support targeting .NET Core 3.0. Either target .NET Core 2.2 or lower, or use a version of the .NET SDK that supports .NET Core 3.0.

[nigel-sampson]

Having some issues updating the pipeline.

##[error]C:\Users\VssAdministrator.nuget\packages\msbuild.sdk.extras\2.0.31\Sdk\Sdk.props(16,5): Error MSB4184: The expression "[System.IO.Path]::GetDirectoryName('')" cannot be evaluated. The path is not of a legal form.

Will work through it some more tonight.

[nigel-sampson]

Glad we managed to get there together

[nigel-sampson]

Thanks for the hard work @mjuen

[mjuen]

No problem :)

It is truly nerve wracking when you assume that the pipeline works after fixing several errors and then the next one appears.

[nigel-sampson]

Hate having to fix pipelines, leaves a lot of terrible commit history.