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/Help/CustomHelpProvider.cs

@@ -0,0 +1,34 @@
+using Spectre.Console.Rendering;
+
+namespace Spectre.Console.Cli.Tests.Data.Help;
+
+internal class CustomHelpProvider : HelpProvider
+{
+    private readonly string version;
+
+    public CustomHelpProvider(ICommandAppSettings settings, string version)
+        : base(settings)
+    {
+        this.version = version;
+    }
+
+    public override IEnumerable<IRenderable> GetHeader(ICommandModel model, ICommandInfo command)
+    {
+        return new IRenderable[]
+        {
+            new Text("--------------------------------------"), Text.NewLine,
+            new Text("---      CUSTOM HELP PROVIDER      ---"), Text.NewLine,
+            new Text("--------------------------------------"), Text.NewLine,
+            Text.NewLine,
+        };
+    }
+
+    public override IEnumerable<IRenderable> GetFooter(ICommandModel model, ICommandInfo command)
+    {
+        return new IRenderable[]
+        {
+            Text.NewLine,
+            new Text($"Version {version}"),
+        };
+    }
+}

src/Tests/Spectre.Console.Cli.Tests/Data/Help/RedirectHelpProvider.cs

@@ -0,0 +1,21 @@
+using Spectre.Console.Rendering;
+
+namespace Spectre.Console.Cli.Tests.Data.Help;
+
+internal class RedirectHelpProvider : IHelpProvider
+{
+    public virtual IEnumerable<IRenderable> Write(ICommandModel model)
+    {
+        return Write(model, null);
+    }
+#nullable enable
+    public virtual IEnumerable<IRenderable> Write(ICommandModel model, ICommandInfo? command)
+#nullable disable
+    {
+        return new[]
+        {
+            new Text("Help has moved online. Please see: http://www.example.com"),
+            Text.NewLine,
+        };
+    }
+}

src/Tests/Spectre.Console.Cli.Tests/Data/Help/RenderMarkupHelpProvider.cs

@@ -0,0 +1,11 @@
+namespace Spectre.Console.Cli.Tests.Data.Help;
+
+internal class RenderMarkupHelpProvider : HelpProvider
+{
+    protected override bool RenderMarkupInline { get; } = true;
+
+    public RenderMarkupHelpProvider(ICommandAppSettings settings)
+        : base(settings)
+    {
+    }
+}