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-engine/src/eval.rs

@@ -21,9 +21,7 @@ pub fn eval_call<D: DebugContext>(
     call: &Call,
     input: PipelineData,
 ) -> Result<PipelineData, ShellError> {
-    if nu_utils::ctrl_c::was_pressed(&engine_state.ctrlc) {
-        return Ok(Value::nothing(call.head).into_pipeline_data());
-    }
+    engine_state.signals().check(call.head)?;
     let decl = engine_state.get_decl(call.decl_id);
 
     if !decl.is_known_external() && call.named_iter().any(|(flag, _, _)| flag.item == "help") {