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/formats/to/text.rs

@@ -3,7 +3,7 @@ use nu_protocol::{
     ast::Call,
     engine::{Command, EngineState, Stack},
     format_duration, format_filesize_from_conf, Category, Config, Example, IntoPipelineData,
-    PipelineData, ShellError, Signature, Value,
+    PipelineData, ShellError, Signature, Type, Value,
 };
 
 #[derive(Clone)]
@@ -15,7 +15,9 @@ impl Command for ToText {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("to text").category(Category::Formats)
+        Signature::build("to text")
+            .input_output_types(vec![(Type::Any, Type::String)])
+            .category(Category::Formats)
     }
 
     fn usage(&self) -> &str {