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/Help/ICommandParameter.cs

@@ -0,0 +1,32 @@
+namespace Spectre.Console.Cli.Help;
+
+/// <summary>
+/// Represents a command parameter.
+/// </summary>
+public interface ICommandParameter
+{
+    /// <summary>
+    /// Gets a value indicating whether the parameter is a flag.
+    /// </summary>
+    bool IsFlag { get; }
+
+    /// <summary>
+    /// Gets a value indicating whether the parameter is required.
+    /// </summary>
+    bool Required { get; }
+
+    /// <summary>
+    /// Gets the description of the parameter.
+    /// </summary>
+    string? Description { get; }
+
+    /// <summary>
+    /// Gets the default value of the parameter, if specified.
+    /// </summary>
+    DefaultValueAttribute? DefaultValue { get; }
+
+    /// <summary>
+    /// Gets a value indicating whether the parameter is hidden.
+    /// </summary>
+    bool IsHidden { get; }
+}