add input_output_type to ansi command (#9817)

# Description

This PR fixes this not working `ansi --list | columns`. I originally
thought that this was a problem with `columns` but it turned out to be a
problem with the input output type of `ansi`. Since `ansi` was only
allowed to return strings, `columns` thought it was getting a string,
but it was a table.

closes #9808

tracking #9812

# User-Facing Changes
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crates/nu-command/src/platform/ansi/ansi_.rs

@@ -507,7 +507,9 @@ impl Command for AnsiCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("ansi")
-            .input_output_types(vec![(Type::Nothing, Type::String)])
+            .input_output_types(vec![
+                (Type::Nothing, Type::String),
+                (Type::Nothing, Type::Table(vec![]))])
             .optional(
                 "code",
                 SyntaxShape::Any,
@@ -524,6 +526,7 @@ impl Command for AnsiCommand {
                 Some('o'),
             )
             .switch("list", "list available ansi code names", Some('l'))
+            .allow_variants_without_examples(true)
             .category(Category::Platform)
     }
 

crates/nu-command/tests/commands/platform/ansi_.rs

@@ -10,3 +10,14 @@ fn test_ansi_shows_error_on_escape() {
 
     assert!(actual.err.contains("no need for escape characters"))
 }
+
+#[test]
+fn test_ansi_list_outputs_table() {
+    let actual = nu!(pipeline(
+        r#"
+            ansi --list | length
+        "#
+    ));
+
+    assert_eq!(actual.out, "424");
+}