Add ctrl_c to RunnablePerItemContext. (#1239)

Also, this commit makes `ls` a per-item command.

A command that processes things item by item may still take some time to stream
out the results from a single item. For example, `ls` on a directory with a lot
of files could be interrupted in the middle of showing all of these files.

tests/commands/ls.rs

@@ -69,3 +69,29 @@ fn lists_regular_files_using_question_mark_wildcard() {
         assert_eq!(actual, "3");
     })
 }
+
+#[test]
+fn lists_all_files_in_directories_from_stream() {
+    Playground::setup("ls_test_4", |dirs, sandbox| {
+        sandbox.mkdir("dir_a").mkdir("dir_b").with_files(vec![
+            EmptyFile("root1.txt"),
+            EmptyFile("root2.txt"),
+            EmptyFile("dir_a/yehuda.10.txt"),
+            EmptyFile("dir_a/jonathan.10.txt"),
+            EmptyFile("dir_b/andres.10.txt"),
+            EmptyFile("dir_b/chicken_not_to_be_picked_up.100.txt"),
+        ]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                echo dir_a dir_b
+                | ls $it
+                | count
+                | echo $it
+            "#
+        ));
+
+        assert_eq!(actual, "4");
+    })
+}