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/math/avg.rs

@@ -2,7 +2,7 @@ use crate::math::reducers::{reducer_for, Reduce};
 use crate::math::utils::run_with_function;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, ShellError, Signature, Span, Value};
+use nu_protocol::{Category, Example, PipelineData, ShellError, Signature, Span, Type, Value};
 
 #[derive(Clone)]
 pub struct SubCommand;
@@ -13,11 +13,13 @@ impl Command for SubCommand {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("math avg").category(Category::Math)
+        Signature::build("math avg")
+            .input_output_types(vec![(Type::List(Box::new(Type::Number)), Type::Number)])
+            .category(Category::Math)
     }
 
     fn usage(&self) -> &str {
-        "Finds the average of a list of numbers or tables"
+        "Returns the average of a list of numbers"
     }
 
     fn search_terms(&self) -> Vec<&str> {
@@ -36,7 +38,7 @@ impl Command for SubCommand {
 
     fn examples(&self) -> Vec<Example> {
         vec![Example {
-            description: "Get the average of a list of numbers",
+            description: "Compute the average of a list of numbers",
             example: "[-50 100.0 25] | math avg",
             result: Some(Value::Float {
                 val: 25.0,