Use memoized I18n.locale_available? in Config.locale

I18n.locale_available? checks a memoized set of locales, unlike
I18n.available_locales which rebuilds its array on every call, so the
common case (the current I18n locale is available) no longer allocates.

Benchmark (Ruby 3.4.9, arm64-darwin25, benchmark-ips):

    require 'benchmark/ips'
    require 'faker'

    Benchmark.ips do |x|
      x.config(warmup: 1, time: 2)
      x.report('Config.locale') { Faker::Config.locale }
    end

Results:

    main:        Config.locale  178.877k (+/-16.1%) i/s
    this commit: Config.locale  992.950k (+/- 2.5%) i/s  (~5.5x)

Author: connorshea

lib/faker.rb

@@ -24,7 +24,10 @@ def locale=(new_locale)
 
       def locale
         # Because I18n.locale defaults to :en, if we don't have :en in our available_locales, errors will happen
-        Thread.current[:faker_config_locale] || @default_locale || (I18n.available_locales.include?(I18n.locale) ? I18n.locale : I18n.available_locales.first)
+        # I18n.locale_available? is used here because it relies on a memoized
+        # set of locales, unlike I18n.available_locales which is recomputed
+        # on every call.
+        Thread.current[:faker_config_locale] || @default_locale || (I18n.locale_available?(I18n.locale) ? I18n.locale : I18n.available_locales.first)
       end
 
       def own_locale