Handle synchronized update control sequences

This will conceal the following message:
erresc: unknown private set/reset mode 2026

See also:
https://gitlab.freedesktop.org/terminal-wg/specifications/-/merge_requests/2#note_579597
microsoft/terminal#8331
dankamongmen/notcurses#1582
xtermjs/xterm.js#3375
kovidgoyal/kitty@5768c54
https://gist.github.com/christianparpart/d8a62cc1ab659194337d73e399004036

st.c

@@ -1568,6 +1568,8 @@ tsetmode(int priv, int set, const int *args, int narg)
 				      and can be mistaken for other control
 				      codes. */
 				break;
+			case 2026: /* TODO: begin/end synchronized update */
+				break;
 			case 7727: /* TODO: application escape mode */
 				break;
 			case 7786: /* TODO: mouse wheel - cursor key translation */