bugfix: add "to yml" command (#15254)

# Description
This fixes #15240, which can be closed after merge.

# User-Facing Changes
- user get now use `to yml` -> exactly the same as `to yaml`


![2025-03-06_00h01_27](https://github.com/user-attachments/assets/e002a96a-26dd-4f9c-9b45-b456a95be158)

# Tests + Formatting
Cargo fmt and clippy 🆗 
I added a test in the only place I could find where `to yaml` was
already tested.

I didn't see the `save.rs::convert_to_extension` function tested
anywhere, but maybe I missed it.

# After Submitting

Not sure this needs an update on the documentation ❓ What do you
suggest?

---------

Co-authored-by: Stefan Holderbach <sholderbach@users.noreply.github.com>

crates/nu-command/tests/format_conversions/yaml.rs

@@ -15,6 +15,21 @@ fn table_to_yaml_text_and_from_yaml_text_back_into_table() {
     assert_eq!(actual.out, "nushell");
 }
 
+#[test]
+fn table_to_yml_text_and_from_yml_text_back_into_table() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            open appveyor.yml
+            | to yml
+            | from yml
+            | get environment.global.PROJECT_NAME
+        "#
+    ));
+
+    assert_eq!(actual.out, "nushell");
+}
+
 #[test]
 fn convert_dict_to_yaml_with_boolean_key() {
     let actual = nu!(pipeline(