Obsolete the AnsiConsoleFactory class (#585)

`AnsiConsoleFactory` should be an internal static class. Creating an `IAnsiConsole` can be achieved through `Spectre.Console.AnsiConsole.Create(AnsiConsoleSettings)`

Since `AnsiConsoleFactory` is public, it can't be changed from a non static class to a static class so obsoleting it is the second best thing to do.

Eventually, when `AnsiConsoleFactory` becomes internal and static, `AnsiConsole.Create` can be simplified to this (and the private static field `_factory` can be removed)

```
public static IAnsiConsole Create(AnsiConsoleSettings settings)
{
    return AnsiConsoleFactory.Create(settings);
}
```

src/Spectre.Console.Testing/TestConsole.cs

@@ -56,8 +56,7 @@ public sealed class TestConsole : IAnsiConsole, IDisposable
         Input = new TestConsoleInput();
         EmitAnsiSequences = false;
 
-        var factory = new AnsiConsoleFactory();
-        _console = factory.Create(new AnsiConsoleSettings
+        _console = AnsiConsole.Create(new AnsiConsoleSettings
         {
             Ansi = AnsiSupport.Yes,
             ColorSystem = (ColorSystemSupport)ColorSystem.TrueColor,