Automatically generate a file that lists all the exported R funs in the project

## Context

If I want to give my R package to a software dev, they can run `webrcli exportedFuns` (or sthgs like that) and get a list of exported functions they can use. 

The tool probably needs to print it to the console by default, and we can supply a file to export the list to. 

Thanks @pedrocoutinhosilva for the idea