Add `getFunctionKind()`

[fisker]

When I work on unicorn/no-anonymous-default-export, I want to display function kind in error message, but I don't want display the function name default, make sense add getFunctionKind() to just return the function kind? Or maybe make getFunctionNameWithKind return kind and name separately?

[voxpelli]

Since getFunctionNameWithKind is already split up into three clear parts – modifiers, kinds and name – we could easily make a new method that returns those three parts separately and then have change getFunctionNameWithKind to simply concatenate those three together.

Want to do a PR?

Also: I guess you are one of the power users of this module outside of eslint-community, should we add you on as a co-maintainer for this one? Then we can help each other out more? Ping @MichaelDeBoey