Declare input and output types of commands (#6796)

* Add failing test that list of ints and floats is List<Number>

* Start defining subtype relation

* Make it possible to declare input and output types for commands

- Enforce them in tests

* Declare input and output types of commands

* Add formatted signatures to `help commands` table

* Revert SyntaxShape::Table -> Type::Table change

* Revert unnecessary derive(Hash) on SyntaxShape

Co-authored-by: JT <547158+jntrnr@users.noreply.github.com>

crates/nu-command/src/strings/detect_columns.rs

@@ -6,7 +6,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, PipelineData, ShellError, Signature, Span,
-    Spanned, SyntaxShape, Value,
+    Spanned, SyntaxShape, Type, Value,
 };
 
 type Input<'t> = Peekable<CharIndices<'t>>;
@@ -27,6 +27,7 @@ impl Command for DetectColumns {
                 "number of rows to skip before detecting",
                 Some('s'),
             )
+            .input_output_types(vec![(Type::String, Type::Table(vec![]))])
             .switch("no-headers", "don't detect headers", Some('n'))
             .category(Category::Strings)
     }