main/vim: add support for custom configuration file not managed by apk

Currently the vim-configuration file is hardcoded as /etc/vim/vimrc. As this file is managed by apk, I don't really want to make changes to this file. Other distros use a second configuration file which is not managed by a package manager to use for customization, e.g. /etc/vim/vimrc.local.

Could you please append something like this to the shipped vimrc?

" Source a global configuration file if available
if filereadable("/etc/vim/vimrc.local")
  source /etc/vim/vimrc.local
endif