feat(i18n): fallback logic of defaultLocale for i18n routing

[ematipico]

Changes

This PR implement the fallback system for when i18n.defaultLocale and i18n.fallbackControl = "redirect" are present in the configuration.

Addresses PLT-1098

The expected behaviour is that if a user defines the defaultLocale to "en" for example, then any route that has /en in, will be redirected to a location that doesn't have /en:

• /en => /
• /en/blog/slug => /blog/slug
• /en/getting-started => /getting-started
• /blog/en/slug => /blog/slug

The fallback to the default locale assumes that the user doesn't have a folder named after the defaultLocale.

To achieve that, here are the changes I made:

• in side the middleware of i18n, I added some code that checks the presence of the default locale in the URL. If present a redirect is returned by the middleware;
• for the build, I had to do a major refactor. I was having a hard time understanding how the routes should be calculated, so I changed the algorithm's logic.
  • now we create a map of locale => RouteData[], essentially we group the routes via their own locale. Routes that don't belong to a locale belong to the default locale;
  • to create the said map, I create a Set, so I can remove those routes that belong to a locale
  • to calculate fallbackFromRoutes and fallbackToRoutes, we now use said map
  • the added logic considers when a locale does a fallback to defaultLocale. The default locale doesn't have a prefix, so we to fallback to the root, e.g.: if defaultLocale is en, then when visitiring /it/getting-started, we do a redirect to /getting-started, NOT /en/getting-started
  • the rest of the logic is the same.

Testing

I updated the test cases based on the previous assumptions

Docs

N/A for now

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