Add support for custom subcommands (#2814)

* Add support for custom subcommands

* clippy

tests/shell/pipeline/commands/internal.rs

@@ -368,6 +368,18 @@ fn run_custom_command_with_flag_missing() {
     assert_eq!(actual.out, "empty");
 }
 
+#[test]
+fn run_custom_subcommand() {
+    let actual = nu!(
+        cwd: ".",
+        r#"
+        def "str double" [x] { echo $x $x | str collect }; str double bob
+        "#
+    );
+
+    assert_eq!(actual.out, "bobbob");
+}
+
 #[test]
 fn set_variable() {
     let actual = nu!(