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-plugin-test-support/src/lib.rs

@@ -79,7 +79,7 @@
 //!
 //! // #[test]
 //! fn test_lowercase() -> Result<(), ShellError> {
-//!     let input = vec![Value::test_string("FooBar")].into_pipeline_data(None);
+//!     let input = vec![Value::test_string("FooBar")].into_pipeline_data(Span::test_data(), None);
 //!     let output = PluginTest::new("lowercase", LowercasePlugin.into())?
 //!         .eval_with("lowercase", input)?
 //!         .into_value(Span::test_data());