Change browser language detection to only redirect from default locale route

What problem does this feature solve?

When using strategies with prefixes, and detectBrowserLanguage enabled, when user goes directly to a route with non-default locale, it would be better if we didn’t redirect to browser’s locale. The user’s intention, in theory, was to visit the site in that specific locale

Some more context in #451

What does the proposed changes look like?

Only redirect and change locale if first navigation is to route that matches defaultLocale.

That would be a breaking change and would require major version.

<div align="right">_{This feature request is available on Nuxt community (#c301)}</div>

[konr4d]

@laurentdebricon A quick workaround https://github.com/konr4d/nuxt-i18n/commit/37f394dba13c055faae1890d7d2d6997a25904e5

Edit: I was able to make nuxt-i18n redirect from root path only and respect the saved cookie. E.g., a new user from France is initially redirected to /fr per hers/his Accept-Language. Then, every / entry results in a redirect to a locale path based on the value in that cookie. I wish I had time to write a proper non-kludge code and form it into a PR. Alas, for now it lives but here: https://github.com/konr4d/nuxt-i18n/tree/only-root-redirect

[rchl]

Fix released in v6.15.0