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-test-support/tests/lowercase/mod.rs

@@ -1,8 +1,8 @@
 use nu_plugin::*;
 use nu_plugin_test_support::PluginTest;
 use nu_protocol::{
-    Example, IntoInterruptiblePipelineData, LabeledError, PipelineData, ShellError, Signature,
-    Span, Type, Value,
+    Example, IntoInterruptiblePipelineData, LabeledError, PipelineData, ShellError, Signals,
+    Signature, Span, Type, Value,
 };
 
 struct LowercasePlugin;
@@ -53,7 +53,7 @@ impl PluginCommand for Lowercase {
                     // Errors in a stream should be returned as values.
                     .unwrap_or_else(|err| Value::error(err, span))
             },
-            None,
+            &Signals::empty(),
         )?)
     }
 }
@@ -72,7 +72,8 @@ impl Plugin for LowercasePlugin {
 fn test_lowercase_using_eval_with() -> Result<(), ShellError> {
     let result = PluginTest::new("lowercase", LowercasePlugin.into())?.eval_with(
         "lowercase",
-        vec![Value::test_string("HeLlO wOrLd")].into_pipeline_data(Span::test_data(), None),
+        vec![Value::test_string("HeLlO wOrLd")]
+            .into_pipeline_data(Span::test_data(), Signals::empty()),
     )?;
 
     assert_eq!(