patch 8.1.2134: modifier keys are not always recognized

Problem:    Modifier keys are not always recognized.
Solution:   Handle key codes generated by xterm with modifyOtherKeys set.
            Add this to libvterm so we can debug it.
diff --git a/src/libvterm/include/vterm.h b/src/libvterm/include/vterm.h
index 02ea91c..28d0a10 100644
--- a/src/libvterm/include/vterm.h
+++ b/src/libvterm/include/vterm.h
@@ -200,6 +200,7 @@
 
 size_t vterm_output_read(VTerm *vt, char *buffer, size_t len);
 
+int vterm_is_modify_other_keys(VTerm *vt);
 void vterm_keyboard_unichar(VTerm *vt, uint32_t c, VTermModifier mod);
 void vterm_keyboard_key(VTerm *vt, VTermKey key, VTermModifier mod);