use to_lowercase in str downcase (#10850)

# Description
as we can see in the [documentation of
`str.to_lowercase`](https://doc.rust-lang.org/std/primitive.str.html#method.to_lowercase),
not only ASCII symbols have lower and upper variants.

- `str upcase` uses the correct method to convert the string

https://github.com/nushell/nushell/blob/7ac5a01e2f5020a1295697edab727cd62a5844b7/crates/nu-command/src/strings/str_/case/upcase.rs#L93
- `str downcase` incorrectly converts only ASCII characters

https://github.com/nushell/nushell/blob/7ac5a01e2f5020a1295697edab727cd62a5844b7/crates/nu-command/src/strings/str_/case/downcase.rs#L124

this PR uses `str.to_lower_case` instead of `str.to_ascii_lowercase` in
`str downcase`.

# User-Facing Changes
- upcase still works fine
```nushell
~ l> "ὀδυσσεύς" | str upcase
ὈΔΥΣΣΕΎΣ
```
- downcase now works

:point_right: before
```nushell
~ l> "ὈΔΥΣΣΕΎΣ" | str downcase
ὈΔΥΣΣΕΎΣ
```
:point_right: after
```nushell
~ l> "ὈΔΥΣΣΕΎΣ" | str downcase
ὀδυσσεύς
```

# Tests + Formatting
- :green_circle: `toolkit fmt`
- :green_circle: `toolkit clippy`
- :black_circle: `toolkit test`
- :black_circle: `toolkit test stdlib`

adds two tests
- `non_ascii_upcase`
- `non_ascii_downcase`

# After Submitting

crates/nu-command/src/strings/str_/case/downcase.rs

@@ -121,7 +121,7 @@ fn operate(
 
 fn action(input: &Value, head: Span) -> Value {
     match input {
-        Value::String { val, .. } => Value::string(val.to_ascii_lowercase(), head),
+        Value::String { val, .. } => Value::string(val.to_lowercase(), head),
         Value::Error { .. } => input.clone(),
         _ => Value::error(
             ShellError::OnlySupportsThisInputType {

crates/nu-command/tests/commands/str_/mod.rs

@@ -76,6 +76,13 @@ fn downcases() {
     })
 }
 
+#[test]
+fn non_ascii_downcase() {
+    let actual = nu!("'ὈΔΥΣΣΕΎΣ' | str downcase");
+
+    assert_eq!(actual.out, "ὀδυσσεύς");
+}
+
 #[test]
 fn upcases() {
     Playground::setup("str_test_4", |dirs, sandbox| {
@@ -96,6 +103,13 @@ fn upcases() {
     })
 }
 
+#[test]
+fn non_ascii_upcase() {
+    let actual = nu!("'ὀδυσσεύς' | str upcase");
+
+    assert_eq!(actual.out, "ὈΔΥΣΣΕΎΣ");
+}
+
 #[test]
 #[ignore = "Playgrounds are not supported in nu-cmd-extra"]
 fn camelcases() {