feat: merging of locale files with the same name for @intlify/unplugin-vue-i18n/messages

[DanielleHuisman]

Currently, unplugin-vue-i18n doesn't merge keys for locale files with the same name. However, this would be a useful feature to allow splitting up locales over multiple files. For example:

src/locales/user/en.yaml
src/locales/user/nl.yaml
src/locales/group/en.yaml
src/locales/group/nl.yaml

// Should include locales from all subdirectories by locale
import messages from '@intlify/unplugin-vue-i18n/messages'

This issue was also brought up for the deprecated vite-plugin-vue-i18n in #3.

This PR implements the merging behaviour. I chose to use lodash.merge to ensure the deep merging is production-ready code.

To test the new behaviour, the Vite locales were split up into two files. Without the changes to unplugin-vue-i18n/src/index.ts, the tests fail, but with the changes they pass again.

[DanielleHuisman]

@kazupon any chance this can get merged soon?

[kazupon]

sorry for my late reply.
I've just reviewed!
please check it!

[kazupon]

@DanielleHuisman
Thanks!

One more request, this feature is for @intlify/unplugin-vue-i18n/messages only.
so, we need to update docs (README.md)

[DanielleHuisman]

One more request, this feature is for @intlify/unplugin-vue-i18n/messages only. so, we need to update docs (README.md)

I updated README.md with a minimal explanation of the merge feature.

[kazupon]

well done!
Thank you so much!