flutter_plugins supports Dart, Java, and Objective-C formatting. Since we'll be using Swift preferentially for macOS plugins (and probably someday for iOS, even if not yet), we should add support for swift-format too. (There are several Swift formatters, but based on internal discussions that's the one we should use.)

It's a bit fiddly to set up at the moment, but we should at least check if it's on the path and run it if so.