fix(highlight): make Ignore hidden by default #41115

Problem:
Ignore is linked to Normal by default, making the text visible instead
of hidden.

Solution:
Replace the default link with an explicit highlight definition using
ctermfg=0 guifg=bg.

test/functional/ui/cursor_spec.lua

@@ -263,7 +263,7 @@ describe('ui/cursor', function()
         m.attr = { background = Screen.colors.DarkGray }
       end
       if m.id_lm then
-        m.id_lm = 78
+        m.id_lm = 79
         m.attr_lm = {}
       end
     end