Add user-overridable callback for cancelling UCIS input (#5564)

* Add user-overridable callback for cancelling UCIS input

To clean up things from qk_ucis_start_user() for instance.

* restore lost newline to quantum/process_keycode/process_ucis.c

Co-Authored-By: shinmai <aapo.saaristo@gmail.com>

quantum/process_keycode/process_ucis.c

@@ -64,6 +64,10 @@ void qk_ucis_symbol_fallback (void) {
   }
 }
 
+__attribute__((weak))
+void qk_ucis_cancel(void) {
+}
+
 void register_ucis(const char *hex) {
   for(int i = 0; hex[i]; i++) {
     uint8_t kc = 0;
@@ -130,6 +134,7 @@ bool process_ucis (uint16_t keycode, keyrecord_t *record) {
 
     if (keycode == KC_ESC) {
       qk_ucis_state.in_progress = false;
+      qk_ucis_cancel();
       return false;
     }