Fix bug in UC_RMOD, add shift and audio support for UC_MOD/UC_RMOD(#8674)

* Invert UC_MOD/UC_RMOD direction when Shift is held

Also use MOD_MASK_SHIFT in process_rgb.c

* Allow audio to be played for UC_MOD, UC_RMOD keycodes as well

* Fix signedness bug in reverse input mode cycling

* Misc formatting in process_unicode_common.c

* Address clang-format issues

* Make decode_utf8 helper function file-local (static)

quantum/process_keycode/process_rgb.c

@@ -56,7 +56,7 @@ bool process_rgb(const uint16_t keycode, const keyrecord_t *record) {
     // Split keyboards need to trigger on key-up for edge-case issue
     if (!record->event.pressed) {
 #endif
-        uint8_t shifted = get_mods() & (MOD_BIT(KC_LSHIFT) | MOD_BIT(KC_RSHIFT));
+        uint8_t shifted = get_mods() & MOD_MASK_SHIFT;
         switch (keycode) {
             case RGB_TOG:
                 rgblight_toggle();