LoggerFactoryExtension.CreateLogger<T> now returns an ILogger<T>

[sunsided]

As proposed in #312, LoggerFactoryExtensions.CreateLogger<T>() now returns an ILogger<T> instead of an ILogger.

In order to achieve this, I moved Logger<T> from Microsoft.Extensions.Logging to Microsoft.Extensions.Logging.Abstractions. I thought about moving LoggerFactoryExtension to Microsoft.Extensions.Logging instead but that seemed to do more harm than good.

[dnfclas]

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[kichalla]

@lodejard @loudej

[kichalla]

LGTM

@rynowak can you also take a look?

[rynowak]

[kichalla]

d679c78

Thanks for the contribution! @sunsided