Add decimals to int when using into string --decimals (#6085)

* Add decimals to int when using `into string --decimals`

* Add tests for `into string` when converting int with `--decimals`

* Apply formatting

* Merge `into_str` test files

* Comment out unused code and add TODOs

* Use decimal separator depending on system locale

* Add test helper to run closure in different locale

* Add tests for int-to-string conversion using different locales

* Add utils function to get system locale

* Add panic message when locking mutex fails

* Catch and resume panic later to prevent Mutex poisoning when test fails

* Move test to `nu-test-support` to keep `nu-utils` free of `nu-*` dependencies

See https://github.com/nushell/nushell/pull/6085#issuecomment-1193131694

* Rename test support fn `with_fake_locale` to `with_locale_override`

* Move `get_system_locale()` to `locale` module

* Allow overriding locale with special env variable (when not in release)

* Use special env var to override locale during testing

* Allow callback to return a value in `with_locale_override()`

* Allow multiple options in `nu!` macro

* Allow to set locale as `nu!` macro option

* Use new `locale` option of `nu!` macro instead of `with_locale_override`

Using the `locale` options does not lock the `LOCALE_OVERRIDE_MUTEX`
mutex in `nu-test-support::locale_override` but instead calls the `nu`
command directly with the `NU_LOCALE_OVERRIDE` environment variable.
This allows for parallel test excecution.

* Fix: Add option identifier for `cwd` in usage of `nu!` macro

* Rely on `Display` trait for formatting `nu!` macro command

- Removed the `DisplayPath` trait
- Implement `Display` for `AbsolutePath`, `RelativePath` and
  `AbsoluteFile`

* Default to locale `en_US.UTF-8` for tests when using `nu!` macro

* Add doc comment to `nu!` macro

* Format code using `cargo fmt --all`

* Pass function directly instead of wrapping the call in a closure

https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure

* Pass function to `or_else()` instead of calling it inside `or()`

https://rust-lang.github.io/rust-clippy/master/index.html#or_fun_call

* Fix: Add option identifier for `cwd` in usage of `nu!` macro

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

@@ -183,3 +183,87 @@ fn from_error() {
 
     assert_eq!(actual.out, "nu::shell::name_not_found");
 }
+
+#[test]
+fn int_into_string() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        10 | into string
+        "#
+    ));
+
+    assert_eq!(actual.out, "10");
+}
+
+#[test]
+fn int_into_string_decimals_0() {
+    let actual = nu!(
+        locale: "en_US.UTF-8",
+        pipeline(
+            r#"
+            10 | into string --decimals 0
+            "#
+        )
+    );
+
+    assert_eq!(actual.out, "10");
+}
+
+#[test]
+fn int_into_string_decimals_1() {
+    let actual = nu!(
+        locale: "en_US.UTF-8",
+        pipeline(
+            r#"
+            10 | into string --decimals 1
+            "#
+        )
+    );
+
+    assert_eq!(actual.out, "10.0");
+}
+
+#[test]
+fn int_into_string_decimals_10() {
+    let actual = nu!(
+        locale: "en_US.UTF-8",
+        pipeline(
+            r#"
+            10 | into string --decimals 10
+            "#
+        )
+    );
+
+    assert_eq!(actual.out, "10.0000000000");
+}
+
+#[test]
+fn int_into_string_decimals_respects_system_locale_de() {
+    // Set locale to `de_DE`, which uses `,` (comma) as decimal separator
+    let actual = nu!(
+        locale: "de_DE.UTF-8",
+        pipeline(
+            r#"
+            10 | into string --decimals 1
+            "#
+        )
+    );
+
+    assert_eq!(actual.out, "10,0");
+}
+
+#[test]
+fn int_into_string_decimals_respects_system_locale_en() {
+    // Set locale to `en_US`, which uses `.` (period) as decimal separator
+    let actual = nu!(
+        locale: "en_US.UTF-8",
+        pipeline(
+            r#"
+            10 | into string --decimals 1
+            "#
+        )
+    );
+
+    assert_eq!(actual.out, "10.0");
+}