Move all fluent configuration extensions into NLog.Config-namespace to avoid conflicts

[snakefoot]

I'm guessing that most new application will go for Microsoft.Extensions.Logging as their ILogger-abstraction.

This means that one only setup NLog during startup-configuration, so it can be combined with Microsoft.Extension.Logging with these usings:

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using NLog;

Now one will get a compiler error if trying to use an ILogger during startup-configuration, because there is one in NLog-namespace and Microsoft.Extensions.Logging-namespace. To avoid issues then on can use var in var logger = loggerFactory.CreateLogger("Startup");

By moving NLog-Setup-extension-methods into NLog.Config-namespace, then one can do this, and not be hit on the head by the compiler:

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using NLog.Config;