feat: commandline edit --accept to instantly execute command (#16193)

# Description

Fixes 
- #11065

Revised 
- #15092

Depends on

- https://github.com/nushell/reedline/pull/933

Adds the `--accept` flag to the `commandline` command, to immediately
accept the input.

Example use case: atuin

# User-facing changes

Users get the ability to pass `--accept` or `-A` to `commandline edit`
in order to immediately execute the resulting commandline.

# Tests + Formatting

I added two test cases that execute `commandline edit -A`.
There is also some documentation about their unintuitive expectations
output.

# After Submitting

The [docs](https://www.nushell.sh/commands/docs/commandline_edit.html)
can be updated to the new flag:

--accept, -A: immediately execute the command (no additional return
required)

> [!NOTE]
>
> This PR will be revised if / when
https://github.com/nushell/reedline/pull/933 is accepted

# Demo


[![asciicast](https://asciinema.org/a/Ql4r1oWu8J0C4MecmIZjxw5Pg.svg)](https://asciinema.org/a/Ql4r1oWu8J0C4MecmIZjxw5Pg)

crates/nu-cli/src/commands/commandline/edit.rs

@@ -26,6 +26,11 @@ impl Command for CommandlineEdit {
                 "replaces the current contents of the buffer (default)",
                 Some('r'),
             )
+            .switch(
+                "accept",
+                "immediately executes the result after edit",
+                Some('A'),
+            )
             .required(
                 "str",
                 SyntaxShape::String,
@@ -61,6 +66,9 @@ impl Command for CommandlineEdit {
             repl.buffer = str;
             repl.cursor_pos = repl.buffer.len();
         }
+
+        repl.accept = call.has_flag(engine_state, stack, "accept")?;
+
         Ok(Value::nothing(call.head).into_pipeline_data())
     }
 }

crates/nu-cli/src/repl.rs

@@ -491,7 +491,9 @@ fn loop_iteration(ctx: LoopContext) -> (bool, Stack, Reedline) {
         // CLEAR STACK-REFERENCE 1
         .with_highlighter(Box::<NoOpHighlighter>::default())
         // CLEAR STACK-REFERENCE 2
-        .with_completer(Box::<DefaultCompleter>::default());
+        .with_completer(Box::<DefaultCompleter>::default())
+        // Ensure immediately accept is always cleared
+        .with_immediately_accept(false);
 
     // Let's grab the shell_integration configs
     let shell_integration_osc2 = config.shell_integration.osc2;
@@ -671,7 +673,7 @@ fn loop_iteration(ctx: LoopContext) -> (bool, Stack, Reedline) {
                 run_shell_integration_reset_application_mode();
             }
 
-            flush_engine_state_repl_buffer(engine_state, &mut line_editor);
+            line_editor = flush_engine_state_repl_buffer(engine_state, line_editor);
         }
         Ok(Signal::CtrlC) => {
             // `Reedline` clears the line content. New prompt is shown
@@ -1126,7 +1128,10 @@ fn run_shell_integration_reset_application_mode() {
 ///
 /// Clear the screen and output anything remaining in the EngineState buffer.
 ///
-fn flush_engine_state_repl_buffer(engine_state: &mut EngineState, line_editor: &mut Reedline) {
+fn flush_engine_state_repl_buffer(
+    engine_state: &mut EngineState,
+    mut line_editor: Reedline,
+) -> Reedline {
     let mut repl = engine_state.repl_state.lock().expect("repl state mutex");
     line_editor.run_edit_commands(&[
         EditCommand::Clear,
@@ -1136,8 +1141,13 @@ fn flush_engine_state_repl_buffer(engine_state: &mut EngineState, line_editor: &
             select: false,
         },
     ]);
+    if repl.accept {
+        line_editor = line_editor.with_immediately_accept(true)
+    }
+    repl.accept = false;
     repl.buffer = "".to_string();
     repl.cursor_pos = 0;
+    line_editor
 }
 
 ///