Add --upgrade switch for mv command (#13505)

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# Description
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Add `--upgrade, -u` switch for `mv` command, corresponding to `cp`.

Closes #13458.

# User-Facing Changes
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```plain
❯ help mv | find update
╭──────────┬───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╮
│        0 │   -u, --update - move and overwite only when the SOURCE file is newer than the destination file or when the destination file is missing       │
╰──────────┴───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
```

# Tests + Formatting
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P.S.  
The standard test kit (`nu-test-support`) doesn't provide utility to
create file with modification timestamp, and I didn't find any test for
this in `cp` command. I had tested on my local machine but I'm not sure
how to integrate it into ci. If unit testing is required, I may need
your guidance.

# After Submitting
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- [x] Command docs are auto generated.

crates/nu-command/src/filesystem/umv.rs

@@ -30,6 +30,11 @@ impl Command for UMv {
                 example: "mv test.txt my/subdirectory",
                 result: None,
             },
+            Example {
+                description: "Move only if source file is newer than target file",
+                example: "mv -u new/test.txt old/",
+                result: None,
+            },
             Example {
                 description: "Move many files into a directory",
                 example: "mv *.txt my/subdirectory",
@@ -49,6 +54,11 @@ impl Command for UMv {
             .switch("verbose", "explain what is being done.", Some('v'))
             .switch("progress", "display a progress bar", Some('p'))
             .switch("interactive", "prompt before overwriting", Some('i'))
+            .switch(
+                "update",
+                "move and overwrite only when the SOURCE file is newer than the destination file or when the destination file is missing",
+                Some('u')
+            )
             .switch("no-clobber", "do not overwrite an existing file", Some('n'))
             .rest(
                 "paths",
@@ -77,6 +87,11 @@ impl Command for UMv {
         } else {
             uu_mv::OverwriteMode::Force
         };
+        let update = if call.has_flag(engine_state, stack, "update")? {
+            UpdateMode::ReplaceIfOlder
+        } else {
+            UpdateMode::ReplaceAll
+        };
 
         #[allow(deprecated)]
         let cwd = current_dir(engine_state, stack)?;
@@ -164,7 +179,7 @@ impl Command for UMv {
             verbose,
             suffix: String::from("~"),
             backup: BackupMode::NoBackup,
-            update: UpdateMode::ReplaceAll,
+            update,
             target_dir: None,
             no_target_dir: false,
             strip_slashes: false,