Support inner classes for groovy

[jevanlingen]

What's changed?

Inner classes our now totally supported (including anonymous classes).

What's your motivation?

• Support for inner classes in Groovy #4063

Anything in particular you'd like reviewers to focus on?

Most work to support inner classes was already done, the new code just adds some missing pieces.

Any additional context

To support

• Groovy parser fails to parse constructor declarations #4704

a visitConstructor was added. The code was 95% the same as the visitMethod. So I merged the visitMethod and visitConstructor again (a ConstructorNode extends a MethodNode, so visitMethod is enough).

Checklist

• I've added unit tests to cover both positive and negative cases
• I've read and applied the recipe conventions and best practices
• I've used the IntelliJ IDEA auto-formatter on affected files