Treat arrow functions in the same way as explicit functions

[antoinetissier]

Whether I implement my function as const myFunction = () => {} or function myFunction(){}, I expect it to be documented in the same way.
For example, getParentPaths should end up in the "Functions" page instead of the "Variables" page, and its signature should be properly documented.

Could be fixed at the source by microsoft/rushstack#1629