Add support for `locale` value objects

[viralpraxis]

We use custom classes (value objects) to represent application-level locales:

# frozen_string_literal: true

class Locale
  attr_reader :code

  delegate :to_sym, to: :code

  def self.[](code)
    new(code: code)
  end

  def initialize(code:)
    @code = code.to_sym
  end

  ...
end

And pass it to I18n: I18n.t(..., locale: Locale[:en]).

While this approach is quite handy (there are, actually, additional Locale attributes; it can be integrated to ActiveRecord and ActiveModel and what not), it's a bit flaky (it depends on MRI implementation details) -- it works on 2.7.4 and does not on 3.0.0.

Seems like the only problem is method

def enforce_available_locales!(locale)
  if locale != false && config.enforce_available_locales
    raise I18n::InvalidLocale.new(locale) if !locale_available?(locale)
   end
end

which can be enhanced to be able to handle wrappers that respond to #to_sym, i.e.

def enforce_available_locales!(locale)
  if locale != false && config.enforce_available_locales
    raise I18n::InvalidLocale.new(locale) if !locale_available?(locale.to_sym)
  end
end