ListStream touchup (#12524)

# Description

Does some misc changes to `ListStream`:
- Moves it into its own module/file separate from `RawStream`.
- `ListStream`s now have an associated `Span`.
- This required changes to `ListStreamInfo` in `nu-plugin`. Note sure if
this is a breaking change for the plugin protocol.
- Hides the internals of `ListStream` but also adds a few more methods.
- This includes two functions to more easily alter a stream (these take
a `ListStream` and return a `ListStream` instead of having to go through
the whole `into_pipeline_data(..)` route).
  -  `map`: takes a `FnMut(Value) -> Value`
  - `modify`: takes a function to modify the inner stream.

crates/nu-command/src/help/help_operators.rs

@@ -21,7 +21,7 @@ impl Command for HelpOperators {
 
     fn run(
         &self,
-        engine_state: &EngineState,
+        _engine_state: &EngineState,
         _stack: &mut Stack,
         call: &Call,
         _input: PipelineData,
@@ -43,9 +43,7 @@ impl Command for HelpOperators {
             ));
         }
 
-        Ok(recs
-            .into_iter()
-            .into_pipeline_data(engine_state.ctrlc.clone()))
+        Ok(Value::list(recs, head).into_pipeline_data())
     }
 }