Replace `mergeDeep` with `deepCopy` from `@intlify/shared`

### Reporting a bug?

I might be mistaken but the [`mergeDeep` function](https://github.com/intlify/bundle-tools/blob/5ed237c9c95711283956be681f51c605e6507df7/packages/unplugin-vue-i18n/src/core/resource.ts#L570) looks similar to (maybe an [older version](https://github.com/intlify/vue-i18n/blame/1556eebf9b362949dda8e51119b30f0f98079314/packages/vue-i18n/src/composer.ts#L972) of) the [`deepCopy` function](https://github.com/intlify/vue-i18n/blob/f491dc6e2fd5ccbb6536adda370a0e5402ff4c1a/packages/shared/src/messages.ts#L5) exported by `@intlify/shared`. Replacing it should make it easier to maintain across packages and possibly prevent issues that we have already solved (such as https://github.com/intlify/vue-i18n/pull/1947).

### Expected behavior

-

### Reproduction

-

### Issue Package

unplugin-vue-i18n

### System Info

```shell
-
```

### Screenshot

_No response_

### Additional context

_No response_

### Validations

- [X] Read the [Contributing Guidelines](https://github.com/intlify/bundle-tools/blob/main/.github/contributing.md).
- [X] Read the README
- [X] Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
- [X] Check that this is a concrete bug. For Q&A open a [GitHub Discussion](https://github.com/intlify/bundle-tools/discussions).