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-utils/src/ctrl_c.rs

@@ -1,13 +0,0 @@
-use std::sync::{
-    atomic::{AtomicBool, Ordering},
-    Arc,
-};
-
-/// Returns true if Nu has received a SIGINT signal / ctrl+c event
-pub fn was_pressed(ctrlc: &Option<Arc<AtomicBool>>) -> bool {
-    if let Some(ctrlc) = ctrlc {
-        ctrlc.load(Ordering::SeqCst)
-    } else {
-        false
-    }
-}

crates/nu-utils/src/lib.rs

@@ -1,5 +1,4 @@
 mod casing;
-pub mod ctrl_c;
 mod deansi;
 pub mod emoji;
 pub mod filesystem;