add -u flag to cp, mv command (#9214)

# Description
Closes: #7853

I found that I want this feature too...

So I take over it, sorry for that @VincenzoCarlino 

# User-Facing Changes

# Tests + Formatting
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crates/nu-command/tests/commands/cp.rs

@@ -1,7 +1,7 @@
 use nu_test_support::fs::file_contents;
 use nu_test_support::fs::{
     files_exist_at, AbsoluteFile,
-    Stub::{EmptyFile, FileWithPermission},
+    Stub::{EmptyFile, FileWithContent, FileWithPermission},
 };
 use nu_test_support::nu;
 use nu_test_support::playground::Playground;
@@ -579,3 +579,33 @@ fn copy_file_with_read_permission_impl(progress: bool) {
         );
     });
 }
+
+#[test]
+fn copy_file_with_update_flag() {
+    copy_file_with_update_flag_impl(false);
+    copy_file_with_update_flag_impl(true);
+}
+
+fn copy_file_with_update_flag_impl(progress: bool) {
+    Playground::setup("cp_test_19", |_dirs, sandbox| {
+        sandbox.with_files(vec![
+            EmptyFile("valid.txt"),
+            FileWithContent("newer_valid.txt", "body"),
+        ]);
+
+        let progress_flag = if progress { "-p" } else { "" };
+
+        let actual = nu!(
+            cwd: sandbox.cwd(),
+            "cp {} -u valid.txt newer_valid.txt; open newer_valid.txt",
+            progress_flag,
+        );
+        assert!(actual.out.contains("body"));
+
+        // create a file after assert to make sure that newest_valid.txt is newest
+        std::thread::sleep(std::time::Duration::from_secs(1));
+        sandbox.with_files(vec![FileWithContent("newest_valid.txt", "newest_body")]);
+        let actual = nu!(cwd: sandbox.cwd(), "cp {} -u newest_valid.txt valid.txt; open valid.txt", progress_flag);
+        assert_eq!(actual.out, "newest_body");
+    });
+}

crates/nu-command/tests/commands/move_/mv.rs

@@ -1,4 +1,4 @@
-use nu_test_support::fs::{files_exist_at, Stub::EmptyFile};
+use nu_test_support::fs::{files_exist_at, Stub::EmptyFile, Stub::FileWithContent};
 use nu_test_support::nu;
 use nu_test_support::playground::Playground;
 
@@ -464,3 +464,25 @@ fn mv_change_case_of_file() {
         assert!(files_in_test_directory.contains(&new_file_name));
     })
 }
+
+#[test]
+fn mv_with_update_flag() {
+    Playground::setup("mv_with_update_flag", |_dirs, sandbox| {
+        sandbox.with_files(vec![
+            EmptyFile("valid.txt"),
+            FileWithContent("newer_valid.txt", "body"),
+        ]);
+
+        let actual = nu!(
+            cwd: sandbox.cwd(),
+            "mv -uf valid.txt newer_valid.txt; open newer_valid.txt",
+        );
+        assert_eq!(actual.out, "body");
+
+        // create a file after assert to make sure that newest_valid.txt is newest
+        std::thread::sleep(std::time::Duration::from_secs(1));
+        sandbox.with_files(vec![FileWithContent("newest_valid.txt", "newest_body")]);
+        let actual = nu!(cwd: sandbox.cwd(), "mv -uf newest_valid.txt valid.txt; open valid.txt");
+        assert_eq!(actual.out, "newest_body");
+    });
+}