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/tests/commands/save.rs

@@ -1,4 +1,4 @@
-use nu_test_support::fs::file_contents;
+use nu_test_support::fs::{file_contents, Stub};
 use nu_test_support::nu;
 use nu_test_support::playground::Playground;
 use std::io::Write;
@@ -149,3 +149,31 @@ fn save_string_and_stream_as_raw() {
         )
     })
 }
+
+#[test]
+fn save_not_override_file_by_default() {
+    Playground::setup("save_test_8", |dirs, sandbox| {
+        sandbox.with_files(vec![Stub::EmptyFile("log.txt")]);
+
+        let actual = nu!(
+            cwd: dirs.root(),
+            r#""abcd" | save save_test_8/log.txt"#
+        );
+        assert!(actual.err.contains("Destination file already exists"));
+    })
+}
+
+#[test]
+fn save_override_works() {
+    Playground::setup("save_test_9", |dirs, sandbox| {
+        sandbox.with_files(vec![Stub::EmptyFile("log.txt")]);
+
+        let expected_file = dirs.test().join("log.txt");
+        nu!(
+            cwd: dirs.root(),
+            r#""abcd" | save save_test_9/log.txt -f"#
+        );
+        let actual = file_contents(expected_file);
+        assert_eq!(actual, "abcd");
+    })
+}