fix: messages `deepCopy` mutates `src` arguments#1947

kazupon merged 2 commits intointlify:masterintlify/vue-i18n:masterfrom

BobbieGoede:fix/message-deep-copy-merging-mutationBobbieGoede/vue-i18n-next:fix/message-deep-copy-merging-mutationCopy head branch name to clipboard