This adds two new options: 'programs.neovim.coc.{enable,settings}`.
These settings offer a simple interface over `xdg.configFile."nvim/coc-settings.json`,
using the standard Nix' syntax instead of a multiline string.
* neovim: allow setting init.vim config alongside plugins
* neovim: add test for neovim plugins
* neovim: make pluginWithConfigType a have type submodule