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/date/list_timezone.rs

@@ -30,17 +30,17 @@ impl Command for SubCommand {
         call: &Call,
         _input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
-        let span = call.head;
+        let head = call.head;
 
         Ok(TZ_VARIANTS
             .iter()
             .map(move |x| {
                 Value::record(
-                    record! { "timezone" => Value::string(x.name(), span) },
-                    span,
+                    record! { "timezone" => Value::string(x.name(), head) },
+                    head,
                 )
             })
-            .into_pipeline_data(engine_state.ctrlc.clone()))
+            .into_pipeline_data(head, engine_state.ctrlc.clone()))
     }
 
     fn examples(&self) -> Vec<Example> {