emulation: ECMA modes should not be saved by DECSC

From esctest:
DECRCTests.test_SaveRestoreCursor_InsertNotAffected
DECSETTiteInhibitTests.test_SaveRestoreCursor_InsertNotAffected
SCORCTests.test_SaveRestoreCursor_InsertNotAffected

Fixes: https://gitlab.gnome.org/GNOME/vte/-/issues/2524
(cherry picked from commit db3c6253)