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/debug/explain.rs

@@ -34,16 +34,14 @@ impl Command for Explain {
         stack: &mut Stack,
         call: &Call,
         _input: PipelineData,
-    ) -> Result<nu_protocol::PipelineData, nu_protocol::ShellError> {
+    ) -> Result<PipelineData, ShellError> {
+        let head = call.head;
         // This was all delightfully stolen from benchmark :)
         let capture_block: Closure = call.req(engine_state, stack, 0)?;
         let block = engine_state.get_block(capture_block.block_id);
-        let ctrlc = engine_state.ctrlc.clone();
         let mut stack = stack.captures_to_stack(capture_block.captures);
-
-        let elements = get_pipeline_elements(engine_state, &mut stack, block, call.head);
-
-        Ok(elements.into_pipeline_data(ctrlc))
+        let elements = get_pipeline_elements(engine_state, &mut stack, block, head);
+        Ok(Value::list(elements, head).into_pipeline_data())
     }
 
     fn examples(&self) -> Vec<Example> {

crates/nu-command/src/debug/metadata_set.rs

@@ -47,13 +47,21 @@ impl Command for MetadataSet {
                 let metadata = PipelineMetadata {
                     data_source: DataSource::FilePath(path.into()),
                 };
-                Ok(input.into_pipeline_data_with_metadata(metadata, engine_state.ctrlc.clone()))
+                Ok(input.into_pipeline_data_with_metadata(
+                    head,
+                    engine_state.ctrlc.clone(),
+                    metadata,
+                ))
             }
             (None, true) => {
                 let metadata = PipelineMetadata {
                     data_source: DataSource::Ls,
                 };
-                Ok(input.into_pipeline_data_with_metadata(metadata, engine_state.ctrlc.clone()))
+                Ok(input.into_pipeline_data_with_metadata(
+                    head,
+                    engine_state.ctrlc.clone(),
+                    metadata,
+                ))
             }
             _ => Err(ShellError::IncorrectValue {
                 msg: "Expected either --datasource-ls(-l) or --datasource-filepath(-f)".to_string(),