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/src/lib.rs

@@ -8,8 +8,8 @@
 //! 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;
@@ -60,7 +60,7 @@
 //!                     // Errors in a stream should be returned as values.
 //!                     .unwrap_or_else(|err| Value::error(err, span))
 //!             },
-//!             None,
+//!             &Signals::empty(),
 //!         )?)
 //!     }
 //! }
@@ -83,7 +83,7 @@
 //!
 //! // #[test]
 //! fn test_lowercase() -> Result<(), ShellError> {
-//!     let input = vec![Value::test_string("FooBar")].into_pipeline_data(Span::test_data(), None);
+//!     let input = vec![Value::test_string("FooBar")].into_pipeline_data(Span::test_data(), Signals::empty());
 //!     let output = PluginTest::new("lowercase", LowercasePlugin.into())?
 //!         .eval_with("lowercase", input)?
 //!         .into_value(Span::test_data())?;