feat(messages)!: graduate the 'msgsep' feature

The old behaviour (e.g. via `set display-=msgsep`) will not be available.
Assuming that messages always are being drawn on msg_grid
(or not drawn at all, and forwarded to `ext_messages` enabled UI)
will allows some simplifcations and enhancements moving forward.

src/nvim/options.lua

@@ -660,7 +660,7 @@ return {
       deny_duplicates=true,
       redraw={'all_windows'},
       varname='p_dy',
-      defaults={if_true="lastline,msgsep"}
+      defaults={if_true="lastline"}
     },
     {
       full_name='eadirection', abbreviation='ead',