Feature: Groovy best practices checks

[fippu82]

I would like to make a feature suggestion: Add a check for Groovy coding best practices. First rule could be to ensure that variables are always defined with at either "def" or explicit type. I got the idea from this blog: https://blogs.sap.com/2017/06/22/avoid-binding-variables-in-groovy-scripts/

[atz-adobson]

@fippu82 Take a look at groovy linting instead https://github.com/nvuillam/npm-groovy-lint

[mwittrock]

Added to feature requests - thank you!