vim-patch:8.2.0839: dropping modifier when putting a character back in typeahead

Problem:    Dropping modifier when putting a character back in typeahead.
Solution:   Add modifier to ins_char_typebuf(). (closes vim/vim#6158)
b42c0d5427

Vim's test doesn't seem to work properly as the hit-enter prompt seems
to be delayed. Add a Lua screen test.

src/nvim/message.c

@@ -1214,7 +1214,7 @@ void wait_return(int redraw)
     } else if (vim_strchr((char_u *)"\r\n ", c) == NULL && c != Ctrl_C) {
       // Put the character back in the typeahead buffer.  Don't use the
       // stuff buffer, because lmaps wouldn't work.
-      ins_char_typebuf(c, mod_mask);
+      ins_char_typebuf(vgetc_char, vgetc_mod_mask);
       do_redraw = true;             // need a redraw even though there is
                                     // typeahead
     }