Stream lazy default output (#15955)

It was brought up in the Discord that `default { open -r foo.txt }`
results in a string instead of streaming output. This changes `default`
such that closures now stream when given simple input.

# Description
If the value isn't expected to be cached, `default` just runs the
closure without caching the value, which allows its output to be
streamed

# User-Facing Changes


# Tests + Formatting
👍 

# After Submitting

crates/nu-command/tests/commands/default.rs

@@ -244,3 +244,9 @@ fn return_closure_value() {
     let actual = nu!(r#"null | default { {||} }"#);
     assert!(actual.out.starts_with("closure"));
 }
+
+#[test]
+fn lazy_output_streams() {
+    let actual = nu!(r#"default { nu --testbin cococo 'hello' } | describe"#);
+    assert!(actual.out.contains("byte stream"));
+}