Use explicit in/out list types for vectorized commands (#9742)

# Description
All commands that declared `.vectorizes_over_list(true)` now also
explicitly declare the list form of their scalar types.

- Explicit in/out list signatures for nu-command
- Explicit in/out list signatures for nu-cmd-extra
- Add comments about cellpath behavior that is still unresolved


# User-Facing Changes
Our type signatures will now be more explicit about which commands
support vectorization over lists.
On the downside this is a bit more verbose and less systematic.

crates/nu-command/src/strings/str_/substring.rs

@@ -42,7 +42,7 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("str substring")
-            .input_output_types(vec![(Type::String, Type::String), (Type::Table(vec![]), Type::Table(vec![]))])
+            .input_output_types(vec![(Type::String, Type::String), (Type::List(Box::new(Type::String)), Type::List(Box::new(Type::String))), (Type::Table(vec![]), Type::Table(vec![]))])
             .vectorizes_over_list(true)
             .allow_variants_without_examples(true)
             .switch(