collect: don't require a closure (#12788)

# Description

This changes the `collect` command so that it doesn't require a closure.
Still allowed, optionally.

Before:

```nushell
open foo.json | insert foo bar | collect { save -f foo.json }
```

After:

```nushell
open foo.json | insert foo bar | collect | save -f foo.json
```

The closure argument isn't really necessary, as collect values are also
supported as `PipelineData`.

# User-Facing Changes
- `collect` command changed

# Tests + Formatting
Example changed to reflect.

# After Submitting
- [ ] release notes
- [ ] we may want to deprecate the closure arg?

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

@@ -245,11 +245,15 @@ impl Command for Save {
                 Ok(PipelineData::empty())
             }
             input => {
-                check_saving_to_source_file(
-                    input.metadata().as_ref(),
-                    &path,
-                    stderr_path.as_ref(),
-                )?;
+                // It's not necessary to check if we are saving to the same file if this is a
+                // collected value, and not a stream
+                if !matches!(input, PipelineData::Value(..) | PipelineData::Empty) {
+                    check_saving_to_source_file(
+                        input.metadata().as_ref(),
+                        &path,
+                        stderr_path.as_ref(),
+                    )?;
+                }
 
                 let bytes =
                     input_to_bytes(input, Path::new(&path.item), raw, engine_state, stack, span)?;