Allow Space Cadet state to be canceled by alternate Shift key

Allows you to press RSHIFT to cancel the insertion of a "(" when holding down LSHIFT. Alternatively, allows you to press LSHIFT to cancel the insertion of a ")" when holding down RSHIFT. This change enables you to renege from outputting a character should you press a shift key erroneously.

quantum/quantum.c

@@ -162,6 +162,10 @@ bool process_record_quantum(keyrecord_t *record) {
         register_mods(MOD_BIT(KC_LSFT));
       }
       else {
+		if (get_mods() & MOD_BIT(KC_RSFT)) {
+		  shift_interrupted[0] = true;
+		  shift_interrupted[1] = true;
+		}
         if (!shift_interrupted[0]) {
           register_code(LSPO_KEY);
           unregister_code(LSPO_KEY);
@@ -178,6 +182,10 @@ bool process_record_quantum(keyrecord_t *record) {
         register_mods(MOD_BIT(KC_RSFT));
       }
       else {
+		if (get_mods() & MOD_BIT(KC_LSFT)) {
+		  shift_interrupted[0] = true;
+		  shift_interrupted[1] = true;
+		}
         if (!shift_interrupted[1]) {
           register_code(RSPC_KEY);
           unregister_code(RSPC_KEY);