Allow all available locales for template lookups.

Following the discussion here:
https://github.com/rails/rails/pull/44174/files#r785160819

Background: The `i18n` gem is relatively lax when it comes
to naming locales. It does not enforce any standard. Thus
it is possible to have e.g. per tenant locales (think
`en_tenant1`, `en_tenant2` etc.). This also worked for
translated templates up until rails 6.1.

Rails 7 changed the template lookup and enforced a naming
scheme for locales. This poses a problem for legacy apps
that use non-standard locale names.

This commit changes the way locale names are detected in
template file names. In addition to the previously used
regexp it also allows all known locales from
`I18n.available_locales`.

This makes it backwards compatible to rails 7.0
behavior while also allowing non-standard locale names.
Thanks to jvillarejo for the great idea.

Also introduce the usage of `Regexp.union`, a wonderful
suggestion by casperisfine.

actionview/lib/action_view/template/resolver.rb

@@ -17,9 +17,11 @@ class PathParser # :nodoc:
       ParsedPath = Struct.new(:path, :details)
 
       def build_path_regex
-        handlers = Template::Handlers.extensions.map { |x| Regexp.escape(x) }.join("|")
-        formats = Template::Types.symbols.map { |x| Regexp.escape(x) }.join("|")
-        locales = "[a-z]{2}(?:[-_][A-Z]{2})?"
+        handlers = Regexp.union(Template::Handlers.extensions.map(&:to_s))
+        formats = Regexp.union(Template::Types.symbols.map(&:to_s))
+        available_locales = I18n.available_locales.map(&:to_s)
+        regular_locales = [/[a-z]{2}(?:[-_][A-Z]{2})?/]
+        locales = Regexp.union(available_locales + regular_locales)
         variants = "[^.]*"
 
         %r{

actionview/test/template/resolver_shared_tests.rb

@@ -242,4 +242,16 @@ def test_finds_template_with_lowdash_format
 
     assert_equal "Texto simple!", es_ar[0].source
   end
+
+  def test_finds_template_with_arbitrarily_formatted_locale
+    I18n.backend.store_translations(:en_customer1, { hello: "hello" })
+    with_file "test/hello_world.en_customer1.text.erb", "Good day, world."
+
+    templates = context.find_all("hello_world", "test", false, [], locale: [:en_customer1])
+
+    assert_equal 1, templates.size
+    assert_equal "Good day, world.", templates[0].source
+  ensure
+    I18n.reload!
+  end
 end