Problem: A plugin does not know when startup scripts were already
triggered. This is useful to determine if a function is
called inside vimrc or after (like when sourcing 'plugin/'
files).
Solution: Add the v:vim_did_init variable (Evgeni Chasnovski)
closes: vim/vim#18668294bce21ee
Nvim has two more steps between sourcing startup scripts and loading
plugins. Set this variable after these two steps.
Co-authored-by: Evgeni Chasnovski <evgeni.chasnovski@gmail.com>