settings: Make external formatter arguments optional (#18340)

If specifying a formatter in the settings like this:

    "languages": {
      "foo": {
        "formatter": {
          "external": {
            "command": "/path/to/foo-formatter"
          }
        }
      }
    }

Zed will show an error like this:

    Invalid user settings file
    data did not match any variant of untagged enum SingleOrVec

This is because the arguments are not optional. The error is hard to
understand, so let's make the arguments actually optional, which makes
the above settings snippet valid.

Release Notes:

- Make external formatter arguments optional

crates/collab/src/tests/integration_tests.rs

@@ -4409,7 +4409,7 @@ async fn test_formatting_buffer(
                 file.defaults.formatter = Some(SelectedFormatter::List(FormatterList(
                     vec![Formatter::External {
                         command: "awk".into(),
-                        arguments: vec!["{sub(/two/,\"{buffer_path}\")}1".to_string()].into(),
+                        arguments: Some(vec!["{sub(/two/,\"{buffer_path}\")}1".to_string()].into()),
                     }]
                     .into(),
                 )));