fix exit_code handling when running a scripts with ctrlc (#11466)

# Description
Fixes: #11394

When run `^sleep 3` we have an `exit_code ListStream`, and when we press
ctrl-c, this `ListStream` will return None. But it's not expected,
because `exit_code` sender in `run_external` always send an exit code
out.

This pr is trying to fix the issue by introducing a `first_guard` into
ListStream, it will always generate a value from underlying stream if
`first_guard` is true, so it's guarantee to have at least one value to
return.

And the pr also do a little refactor, which makes use of
`ListStream::from_stream` rather than construct it manually.

# User-Facing Changes
## Before
```
> nu -c "^sleep 3"  # press ctrl-c
> echo $env.LAST_EXIT_CODE
0
```

## After
```
> nu -c "^sleep 3"  # press ctrl-c
> echo $env.LAST_EXIT_CODE
255
```

# Tests + Formatting
None, sorry that I don't think it's easy to test the ctrlc behavior.

# After Submitting
None

crates/nu-command/src/generators/seq.rs

@@ -140,30 +140,30 @@ pub fn run_seq(
     if !contains_decimals {
         // integers only
         Ok(PipelineData::ListStream(
-            nu_protocol::ListStream {
-                stream: Box::new(IntSeq {
+            nu_protocol::ListStream::from_stream(
+                IntSeq {
                     count: first as i64,
                     step: step as i64,
                     last: last as i64,
                     span,
-                }),
-                ctrlc: engine_state.ctrlc.clone(),
-            },
+                },
+                engine_state.ctrlc.clone(),
+            ),
             None,
         ))
     } else {
         // floats
         Ok(PipelineData::ListStream(
-            nu_protocol::ListStream {
-                stream: Box::new(FloatSeq {
+            nu_protocol::ListStream::from_stream(
+                FloatSeq {
                     first,
                     step,
                     last,
                     index: 0,
                     span,
-                }),
-                ctrlc: engine_state.ctrlc.clone(),
-            },
+                },
+                engine_state.ctrlc.clone(),
+            ),
             None,
         ))
     }