allow help to return a Type::Table (#10082)

# Description

This PR changes the signature of the `help` command so that it can
return a `Type::Table`.

closes #10077 

# User-Facing Changes
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crates/nu-command/src/help/help_.rs

@@ -20,7 +20,10 @@ impl Command for Help {
 
     fn signature(&self) -> Signature {
         Signature::build("help")
-            .input_output_types(vec![(Type::Nothing, Type::String)])
+            .input_output_types(vec![
+                (Type::Nothing, Type::String),
+                (Type::Nothing, Type::Table(vec![])),
+            ])
             .rest(
                 "rest",
                 SyntaxShape::String,