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/terminal.c

@@ -1348,7 +1348,7 @@ static bool send_mouse_event(Terminal *term, int c)
   }
 
 end:
-  ins_char_typebuf(c, mod_mask);
+  ins_char_typebuf(vgetc_char, vgetc_mod_mask);
   return true;
 }