Add support for npm-groovy-lint

I would like to add support for the `npm-groovy-lint` linter as per #246.

There is still the ugliness of having Spotless call Node.js only to then call a separate Java process, but I think that the sugary configuration layer provided by this library is valuable enough. Moreover, I don't think it's worth re-implementing the logic in Java, especially as the linter is under active development (and thus, whoever ported the logic would be forced to always play catch-up or lose out on the new features being provided).

I'm happy to take this work on if there are no objections!

Empieza leyendo el issue #246 y las integraciones existentes de Spotless para averiguar cómo se configuran y se invocan los linters externos. Comprueba cómo llamaría Spotless a npm-groovy-lint mediante Node.js y el proceso Java independiente; se considera terminado cuando el linter está soportado a través de la capa de configuración de la biblioteca con una cobertura adecuada.

Escrito por el modelo de indexación a partir del texto del issue.