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/DogCommand.cs

@@ -0,0 +1,30 @@
+namespace Spectre.Console.Tests.Data;
+
+[Description("The dog command.")]
+public class DogCommand : AnimalCommand<DogSettings>
+{
+    public override ValidationResult Validate(CommandContext context, DogSettings settings)
+    {
+        if (context is null)
+        {
+            throw new System.ArgumentNullException(nameof(context));
+        }
+
+        if (settings is null)
+        {
+            throw new System.ArgumentNullException(nameof(settings));
+        }
+
+        if (settings.Age > 100 && !context.Remaining.Raw.Contains("zombie"))
+        {
+            return ValidationResult.Error("Dog is too old...");
+        }
+
+        return base.Validate(context, settings);
+    }
+
+    public override int Execute(CommandContext context, DogSettings settings)
+    {
+        return 0;
+    }
+}