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/char_.rs

@@ -4,7 +4,7 @@ use lazy_static::lazy_static;
 use nu_engine::CallExt;
 use nu_protocol::{
     ast::Call, engine::Command, Category, Example, IntoInterruptiblePipelineData, IntoPipelineData,
-    PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
+    PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 // Character used to separate directories in a Path Environment variable on windows is ";"
@@ -156,6 +156,7 @@ impl Command for Char {
 
     fn signature(&self) -> Signature {
         Signature::build("char")
+            .input_output_types(vec![(Type::Nothing, Type::String)])
             .optional(
                 "character",
                 SyntaxShape::Any,