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.

src/Spectre.Console.Cli/Internal/Configuration/CommandAppSettings.cs

@@ -4,7 +4,8 @@ internal sealed class CommandAppSettings : ICommandAppSettings
 {
     public string? ApplicationName { get; set; }
     public string? ApplicationVersion { get; set; }
-    public bool ShowOptionDefaultValues { get; set; }
+    public int MaximumIndirectExamples { get; set; }
+    public bool ShowOptionDefaultValues { get; set; }
     public IAnsiConsole? Console { get; set; }
     public ICommandInterceptor? Interceptor { get; set; }
     public ITypeRegistrarFrontend Registrar { get; set; }
@@ -24,7 +25,8 @@ internal sealed class CommandAppSettings : ICommandAppSettings
     {
         Registrar = new TypeRegistrar(registrar);
         CaseSensitivity = CaseSensitivity.All;
-        ShowOptionDefaultValues = true;
+        ShowOptionDefaultValues = true;
+        MaximumIndirectExamples = 5;
     }
 
     public bool IsTrue(Func<CommandAppSettings, bool> func, string environmentVariableName)