Add new command to source functions in active editor

[juliasilge]

Originally posted by @kylebutts in #6640:

This is a feature request to add a command to source all functions in the active editor for R/python. I think this feature would make a function-based workflow much easier. Make changes to some functions, source all functions, and then rerun your code. This information should be easily accessible in ark?

[juliasilge]

This would be similar to what devtools::load_all() does in an R package, and/or similar to the existing "Source R File" command, but only executing the function definitions and not the other code in the file.

@kylebutts notes:

The R extension of neovim has this feature and it's quite nice.