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/strings/format/date.rs

@@ -127,7 +127,7 @@ fn run(
             Some(format) => format_helper(value, format.item.as_str(), format.span, head),
             None => format_helper_rfc2822(value, head),
         },
-        engine_state.ctrlc.clone(),
+        engine_state.signals(),
     )
 }
 

crates/nu-command/src/strings/format/duration.rs

@@ -81,7 +81,7 @@ impl Command for FormatDuration {
             arg,
             input,
             call.head,
-            engine_state.ctrlc.clone(),
+            engine_state.signals(),
         )
     }
 
@@ -108,7 +108,7 @@ impl Command for FormatDuration {
             arg,
             input,
             call.head,
-            working_set.permanent().ctrlc.clone(),
+            working_set.permanent().signals(),
         )
     }
 

crates/nu-command/src/strings/format/filesize.rs

@@ -76,7 +76,7 @@ impl Command for FormatFilesize {
             arg,
             input,
             call.head,
-            engine_state.ctrlc.clone(),
+            engine_state.signals(),
         )
     }
 
@@ -101,7 +101,7 @@ impl Command for FormatFilesize {
             arg,
             input,
             call.head,
-            working_set.permanent().ctrlc.clone(),
+            working_set.permanent().signals(),
         )
     }