add -f, --force for save command (#7262)

# Description

Closes: #6920 

# User-Facing Changes

```
❯ "asdf" | save dump.rdb
Error:
  × Destination file already exists
   ╭─[entry #21:1:1]
 1 │ "asdf" | save dump.rdb
   ·               ────┬───
   ·                   ╰── Destination file '/tmp/dump.rdb' already exists
   ╰────
  help: you can use -f, --force to force overwriting the destination
```
# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

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

@@ -45,6 +45,7 @@ impl Command for Save {
             )
             .switch("raw", "save file as raw binary", Some('r'))
             .switch("append", "append input to the end of the file", Some('a'))
+            .switch("force", "overwrite the destination", Some('f'))
             .category(Category::FileSystem)
     }
 
@@ -57,6 +58,7 @@ impl Command for Save {
     ) -> Result<nu_protocol::PipelineData, nu_protocol::ShellError> {
         let raw = call.has_flag("raw");
         let append = call.has_flag("append");
+        let force = call.has_flag("force");
 
         let span = call.head;
 
@@ -64,7 +66,21 @@ impl Command for Save {
         let arg_span = path.span;
         let path = Path::new(&path.item);
 
-        let file = match (append, path.exists()) {
+        let path_exists = path.exists();
+        if path_exists && !force && !append {
+            return Err(ShellError::GenericError(
+                "Destination file already exists".into(),
+                format!(
+                    "Destination file '{}' already exists",
+                    path.to_string_lossy()
+                ),
+                Some(arg_span),
+                Some("you can use -f, --force to force overwriting the destination".into()),
+                Vec::new(),
+            ));
+        }
+
+        let file = match (append, path_exists) {
             (true, true) => std::fs::OpenOptions::new()
                 .write(true)
                 .append(true)