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/formats/to/text.rs

@@ -1,6 +1,6 @@
 use chrono_humanize::HumanTime;
 use nu_engine::command_prelude::*;
-use nu_protocol::{format_duration, format_filesize_from_conf, Config, ListStream, RawStream};
+use nu_protocol::{format_duration, format_filesize_from_conf, Config, RawStream, ValueIterator};
 
 #[derive(Clone)]
 pub struct ToText;
@@ -41,7 +41,7 @@ impl Command for ToText {
             Ok(PipelineData::ExternalStream {
                 stdout: Some(RawStream::new(
                     Box::new(ListStreamIterator {
-                        stream,
+                        stream: stream.into_inner(),
                         separator: line_ending.into(),
                         config: config.clone(),
                     }),
@@ -86,7 +86,7 @@ impl Command for ToText {
 }
 
 struct ListStreamIterator {
-    stream: ListStream,
+    stream: ValueIterator,
     separator: String,
     config: Config,
 }