Preparations for the 1.0 release

* Less cluttered solution layout.
* Move examples to a repository of its own.
* Move Roslyn analyzer to a repository of its own.
* Enable central package management.
* Clean up csproj files.
* Add README file to NuGet packages.

src/Tests/Spectre.Console.Cli.Tests/Data/Commands/CatCommand.cs

@@ -0,0 +1,9 @@
+namespace Spectre.Console.Tests.Data;
+
+public class CatCommand : AnimalCommand<CatSettings>
+{
+    public override int Execute(CommandContext context, CatSettings settings)
+    {
+        return 0;
+    }
+}