Problem: The shellmenu plugin has to be copied or sourced to be used.
Solution: Turn it into a package.
https://github.com/vim/vim/commit/fead3ac9a35e0fc358141d3eb19574cd8a3ecb55
Justification for changes that do not carry over
- `Filelist` does not exist in neovim
- neovim does not have a `README.txt` in `runtime/macros`
