defaults: background=dark #2894 (#9205)

By historical accident, Nvim defaults to background=light. So on a dark
background, `:colorscheme default` looks completely wrong.

The "smart" logic that Vim uses is confusing for anyone who uses Vim on
multiple platforms, so rather than mimic that, pick the (hopefully) most
common default.

- Since Neovim is dark-powered, we assume most users have dark backgrounds.
- Most of the GUIs tend to have a dark background by default.

ref #6289

src/nvim/options.lua

@@ -132,10 +132,10 @@ return {
     {
       full_name='background', abbreviation='bg',
       type='string', scope={'global'},
-      vi_def=true,
+      vim=true,
       redraw={'all_windows'},
       varname='p_bg',
-      defaults={if_true={vi="light"}}
+      defaults={if_true={vi="light",vim="dark"}}
     },
     {
       full_name='backspace', abbreviation='bs',