fix: fix commandline when called with no arguments (#8207)

# Description

This fixes the `commandline` command when it's run with no arguments, so
it outputs the command being run. New line characters are included.

This allows for:

- [A way to get current command inside pre_execution hook · Issue #6264
· nushell/nushell](https://github.com/nushell/nushell/issues/6264)
- The possibility of *Atuin* to work work *Nushell*. *Atuin* hooks need
to know the current repl input before it is run.

# User-Facing Changes

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-protocol/src/engine/engine_state.rs

@@ -13,7 +13,7 @@ use std::num::NonZeroUsize;
 use std::path::Path;
 use std::path::PathBuf;
 use std::{
-    collections::{HashMap, HashSet, VecDeque},
+    collections::{HashMap, HashSet},
     sync::{
         atomic::{AtomicBool, AtomicU32},
         Arc, Mutex,
@@ -22,15 +22,6 @@ use std::{
 
 static PWD_ENV: &str = "PWD";
 
-// TODO: move to different file? where?
-/// An operation to be performed with the current buffer of the interactive shell.
-#[derive(Debug, Clone)]
-pub enum ReplOperation {
-    Append(String),
-    Insert(String),
-    Replace(String),
-}
-
 /// Organizes usage messages for various primitives
 #[derive(Debug, Clone)]
 pub struct Usage {
@@ -134,8 +125,9 @@ pub struct EngineState {
     pub previous_env_vars: HashMap<String, Value>,
     pub config: Config,
     pub pipeline_externals_state: Arc<(AtomicU32, AtomicU32)>,
-    pub repl_buffer_state: Arc<Mutex<Option<String>>>,
-    pub repl_operation_queue: Arc<Mutex<VecDeque<ReplOperation>>>,
+    pub repl_buffer_state: Arc<Mutex<String>>,
+    // A byte position, as `EditCommand::MoveToPosition` is also a byte position
+    pub repl_cursor_pos: Arc<Mutex<usize>>,
     #[cfg(feature = "plugin")]
     pub plugin_signatures: Option<PathBuf>,
     #[cfg(not(windows))]
@@ -186,8 +178,8 @@ impl EngineState {
             previous_env_vars: HashMap::new(),
             config: Config::default(),
             pipeline_externals_state: Arc::new((AtomicU32::new(0), AtomicU32::new(0))),
-            repl_buffer_state: Arc::new(Mutex::new(None)),
-            repl_operation_queue: Arc::new(Mutex::new(VecDeque::new())),
+            repl_buffer_state: Arc::new(Mutex::new("".to_string())),
+            repl_cursor_pos: Arc::new(Mutex::new(0)),
             #[cfg(feature = "plugin")]
             plugin_signatures: None,
             #[cfg(not(windows))]