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/network/url/decode.rs

@@ -48,7 +48,7 @@ impl Command for SubCommand {
     ) -> Result<PipelineData, ShellError> {
         let cell_paths: Vec<CellPath> = call.rest(engine_state, stack, 0)?;
         let args = CellPathOnlyArgs::from(cell_paths);
-        operate(action, args, input, call.head, engine_state.ctrlc.clone())
+        operate(action, args, input, call.head, engine_state.signals())
     }
 
     fn examples(&self) -> Vec<Example> {

crates/nu-command/src/network/url/encode.rs

@@ -50,15 +50,9 @@ impl Command for SubCommand {
         let cell_paths: Vec<CellPath> = call.rest(engine_state, stack, 0)?;
         let args = CellPathOnlyArgs::from(cell_paths);
         if call.has_flag(engine_state, stack, "all")? {
-            operate(
-                action_all,
-                args,
-                input,
-                call.head,
-                engine_state.ctrlc.clone(),
-            )
+            operate(action_all, args, input, call.head, engine_state.signals())
         } else {
-            operate(action, args, input, call.head, engine_state.ctrlc.clone())
+            operate(action, args, input, call.head, engine_state.signals())
         }
     }