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-plugin-engine/src/context.rs

@@ -3,23 +3,21 @@ use nu_engine::{get_eval_block_with_early_return, get_full_help, ClosureEvalOnce
 use nu_protocol::{
     ast::Call,
     engine::{Closure, EngineState, Redirection, Stack},
-    Config, IntoSpanned, OutDest, PipelineData, PluginIdentity, ShellError, Span, Spanned, Value,
+    Config, IntoSpanned, OutDest, PipelineData, PluginIdentity, ShellError, Signals, Span, Spanned,
+    Value,
 };
 use std::{
     borrow::Cow,
     collections::HashMap,
-    sync::{
-        atomic::{AtomicBool, AtomicU32},
-        Arc,
-    },
+    sync::{atomic::AtomicU32, Arc},
 };
 
 /// Object safe trait for abstracting operations required of the plugin context.
 pub trait PluginExecutionContext: Send + Sync {
     /// A span pointing to the command being executed
     fn span(&self) -> Span;
-    /// The interrupt signal, if present
-    fn ctrlc(&self) -> Option<&Arc<AtomicBool>>;
+    /// The [`Signals`] struct, if present
+    fn signals(&self) -> &Signals;
     /// The pipeline externals state, for tracking the foreground process group, if present
     fn pipeline_externals_state(&self) -> Option<&Arc<(AtomicU32, AtomicU32)>>;
     /// Get engine configuration
@@ -80,8 +78,8 @@ impl<'a> PluginExecutionContext for PluginExecutionCommandContext<'a> {
         self.call.head
     }
 
-    fn ctrlc(&self) -> Option<&Arc<AtomicBool>> {
-        self.engine_state.ctrlc.as_ref()
+    fn signals(&self) -> &Signals {
+        self.engine_state.signals()
     }
 
     fn pipeline_externals_state(&self) -> Option<&Arc<(AtomicU32, AtomicU32)>> {
@@ -234,8 +232,8 @@ impl PluginExecutionContext for PluginExecutionBogusContext {
         Span::test_data()
     }
 
-    fn ctrlc(&self) -> Option<&Arc<AtomicBool>> {
-        None
+    fn signals(&self) -> &Signals {
+        &Signals::EMPTY
     }
 
     fn pipeline_externals_state(&self) -> Option<&Arc<(AtomicU32, AtomicU32)>> {