Add and use new Signals struct (#13314)

# Description
This PR introduces a new `Signals` struct to replace our adhoc passing
around of `ctrlc: Option<Arc<AtomicBool>>`. Doing so has a few benefits:
- We can better enforce when/where resetting or triggering an interrupt
is allowed.
- Consolidates `nu_utils::ctrl_c::was_pressed` and other ad-hoc
re-implementations into a single place: `Signals::check`.
- This allows us to add other types of signals later if we want. E.g.,
exiting or suspension.
- Similarly, we can more easily change the underlying implementation if
we need to in the future.
- Places that used to have a `ctrlc` of `None` now use
`Signals::empty()`, so we can double check these usages for correctness
in the future.

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

@@ -166,7 +166,7 @@ used as the next argument to the closure, otherwise generation stops.
 
         Ok(iter
             .flatten()
-            .into_pipeline_data(call.head, engine_state.ctrlc.clone()))
+            .into_pipeline_data(call.head, engine_state.signals().clone()))
     }
 }
 

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

@@ -129,7 +129,7 @@ pub fn run_seq(
                 span,
             },
             span,
-            engine_state.ctrlc.clone(),
+            engine_state.signals().clone(),
         )
     } else {
         ListStream::new(
@@ -141,7 +141,7 @@ pub fn run_seq(
                 span,
             },
             span,
-            engine_state.ctrlc.clone(),
+            engine_state.signals().clone(),
         )
     };