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/str_/substring.rs

@@ -3,7 +3,7 @@ use nu_engine::CallExt;
 use nu_protocol::ast::Call;
 use nu_protocol::ast::CellPath;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value};
+use nu_protocol::{Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value};
 use std::cmp::Ordering;
 
 #[derive(Clone)]
@@ -38,6 +38,8 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("str substring")
+            .input_output_types(vec![(Type::String, Type::String)])
+            .vectorizes_over_list(true)
             .required(
                 "range",
                 SyntaxShape::Any,