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

[BobbieGoede]

Reporting a bug?

I might be mistaken but the mergeDeep function looks similar to (maybe an older version of) the deepCopy function 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 intlify/vue-i18n#1947).

Expected behavior

Reproduction

Issue Package

unplugin-vue-i18n

System Info

-

Screenshot

No response

Additional context

No response

Validations

• Read the Contributing Guidelines.
• Read the README
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.

[BobbieGoede]

This isn't really a bug (not sure if people are experiencing with deep copy in this plugin) but more of a maintenance issue, I haven't tried reproducing previously found deepCopy issues using the code here.