Add support for npm-groovy-lint

[Alex-Weatherhead]

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!

[nedtwigg]

Very happy to take such a PR! It might be easier for you if you first take a look at one of the following issues.

• #766
• #675
• #934