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.

examples/Cli/Help/DefaultCommand.cs

@@ -0,0 +1,20 @@
+using Spectre.Console;
+using Spectre.Console.Cli;
+
+namespace Help;
+
+public sealed class DefaultCommand : Command
+{
+    private IAnsiConsole _console;
+
+    public DefaultCommand(IAnsiConsole console)
+    {
+        _console = console;
+    }
+
+    public override int Execute(CommandContext context)
+    {
+        _console.WriteLine("Hello world");
+        return 0;
+    }
+}