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

test/functional/ui/screen.lua

@@ -72,6 +72,7 @@
 -- To debug screen tests, see Screen:redraw_debug().
 
 local global_helpers = require('test.helpers')
+local deepcopy = global_helpers.deepcopy
 local shallowcopy = global_helpers.shallowcopy
 local helpers = require('test.functional.helpers')(nil)
 local request, run, uimeths = helpers.request, helpers.run, helpers.uimeths
@@ -176,6 +177,10 @@ function Screen:set_default_attr_ids(attr_ids)
   self._default_attr_ids = attr_ids
 end
 
+function Screen:get_default_attr_ids()
+  return deepcopy(self._default_attr_ids)
+end
+
 function Screen:set_default_attr_ignore(attr_ignore)
   self._default_attr_ignore = attr_ignore
 end