Add support for localized route paths

[ellmetha]

Description

The framework should let users easily configure localized routes. There are two types of localized routes to consider:

1. localized routes whose paths are translated (eg. /cars and /voitures)
2. localized routes whose paths are translated and that are prefixed by the current locale (eg. /en/cars and /fr/voitures)

In order to let users define such routes, let's add the ability to leverage a #localized block when defining routes. For example:

Marten.routes.draw do
  localized(prefix: true) do
    path "routes.article_list", ArticleListHandler, name: "article_list"
    path "routes.article_detail", ArticleDetailHandler, name: "article_detail"
    path "routes.my_app", MyApp::ROUTES, name: "my_app"
  end
end

This method should accept the following arguments:

• prefix - whether or not a locale prefix (eg. /en/ or /fr/ depending on the activated locales) should be used for the generated routes (should default to false)
• prefix_default_locale - whether or not the default locale should be prefixed when prefix: true is used

Using #localized would instruct the routing map that the wrapped path must be localized and that their actual "paths" must be treated as localization paths. For example, the associated translations could look like this:

en:
  routes:
    article_list: /articles
    article_detail: /articles/<pk:int>
    my_app: /my-app

When generating the full URL of localized routes, the associated translations should be used in order to generate the localized URL and (optionally) prefix it with the current locale.