Allow custom help providers (#1259)

Allow custom help providers

* Version option will show in help even with a default command

* Reserve `-v` and `--version` as special Spectre.Console command line arguments (nb. breaking change for Spectre.Console users who have a default command with a settings class that uses either of these switches).

* Help writer correctly determines if trailing commands exist and whether to display them as optional or mandatory in the usage statement.

* Ability to control the number of indirect commands to display in the help text when the command itself doesn't have any examples of its own. Defaults to 5 (for backward compatibility) but can be set to any integer or zero to disable completely.

* Significant increase in unit test coverage for the help writer.

* Minor grammatical improvements to website documentation.

test/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}"),
+        };
+    }
+}

test/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,
+        };
+    }
+}