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/debug/debug_.rs

@@ -46,7 +46,7 @@ impl Command for Debug {
                     Value::string(x.to_expanded_string(", ", &config), head)
                 }
             },
-            engine_state.ctrlc.clone(),
+            engine_state.signals(),
         )
     }
 

crates/nu-command/src/debug/metadata_set.rs

@@ -48,7 +48,7 @@ impl Command for MetadataSet {
         let ds_fp: Option<String> = call.get_flag(engine_state, stack, "datasource-filepath")?;
         let ds_ls = call.has_flag(engine_state, stack, "datasource-ls")?;
         let content_type: Option<String> = call.get_flag(engine_state, stack, "content-type")?;
-
+        let signals = engine_state.signals().clone();
         let metadata = input
             .metadata()
             .clone()
@@ -58,19 +58,15 @@ impl Command for MetadataSet {
         match (ds_fp, ds_ls) {
             (Some(path), false) => Ok(input.into_pipeline_data_with_metadata(
                 head,
-                engine_state.ctrlc.clone(),
+                signals,
                 metadata.with_data_source(DataSource::FilePath(path.into())),
             )),
             (None, true) => Ok(input.into_pipeline_data_with_metadata(
                 head,
-                engine_state.ctrlc.clone(),
+                signals,
                 metadata.with_data_source(DataSource::Ls),
             )),
-            _ => Ok(input.into_pipeline_data_with_metadata(
-                head,
-                engine_state.ctrlc.clone(),
-                metadata,
-            )),
+            _ => Ok(input.into_pipeline_data_with_metadata(head, signals, metadata)),
         }
     }