Split nu-cli into nu-cli/nu-engine (#2898)

We split off the evaluation engine part of nu-cli into its own crate. This helps improve build times for nu-cli by 17% in my tests. It also helps us see a bit better what's the core engine portion vs the part specific to the interactive CLI piece.

There's more than can be done here, but I think it's a good start in the right direction.

crates/nu-stream/src/interruptible.rs

@@ -33,3 +33,16 @@ impl<V> Stream for InterruptibleStream<V> {
         }
     }
 }
+
+pub trait Interruptible<V> {
+    fn interruptible(self, ctrl_c: Arc<AtomicBool>) -> InterruptibleStream<V>;
+}
+
+impl<S, V> Interruptible<V> for S
+where
+    S: Stream<Item = V> + Send + 'static,
+{
+    fn interruptible(self, ctrl_c: Arc<AtomicBool>) -> InterruptibleStream<V> {
+        InterruptibleStream::new(self, ctrl_c)
+    }
+}