Fix trailing slash in PWD set by cd (#12760)

# Description

Fixes #12758.

#12662 introduced a bug where calling `cd` with a path with a trailing
slash would cause `PWD` to be set to a path including a trailing slash,
which is not allowed. This adds a helper to `nu_path` to remove this,
and uses it in the `cd` command to clean it up before setting `PWD`.

# Tests + Formatting
I added some tests to make sure we don't regress on this in the future.

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

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

@@ -26,6 +26,16 @@ fn filesystem_change_from_current_directory_using_relative_path() {
     })
 }
 
+#[test]
+fn filesystem_change_from_current_directory_using_relative_path_with_trailing_slash() {
+    Playground::setup("cd_test_1_slash", |dirs, _| {
+        // Intentionally not using correct path sep because this should work on Windows
+        let actual = nu!( cwd: dirs.root(), "cd cd_test_1_slash/; $env.PWD");
+
+        assert_eq!(PathBuf::from(actual.out), *dirs.test());
+    })
+}
+
 #[test]
 fn filesystem_change_from_current_directory_using_absolute_path() {
     Playground::setup("cd_test_2", |dirs, _| {
@@ -42,6 +52,23 @@ fn filesystem_change_from_current_directory_using_absolute_path() {
     })
 }
 
+#[test]
+fn filesystem_change_from_current_directory_using_absolute_path_with_trailing_slash() {
+    Playground::setup("cd_test_2", |dirs, _| {
+        let actual = nu!(
+            cwd: dirs.test(),
+            r#"
+                cd '{}{}'
+                $env.PWD
+            "#,
+            dirs.formats().display(),
+            std::path::MAIN_SEPARATOR_STR,
+        );
+
+        assert_eq!(PathBuf::from(actual.out), dirs.formats());
+    })
+}
+
 #[test]
 fn filesystem_switch_back_to_previous_working_directory() {
     Playground::setup("cd_test_3", |dirs, sandbox| {