Add suspenders:development:environment generator

To do:

- [ ] Decide which options we want to include from the existing
  generator
- [ ] Manually test on a real app
- [ ] News and readme entries

Reference:
https://github.com/thoughtbot/suspenders/blob/55cb5c246fe53aea4cf53436c7c208b40ad13e85/lib/suspenders/generators/app_generator.rb#L80-L92

lib/generators/suspenders/development/environment_generator.rb

@@ -0,0 +1,11 @@
+module Suspenders
+  module Generators
+    module Development
+      class EnvironmentGenerator < Rails::Generators::Base
+        def raise_on_missing_translations
+          uncomment_lines("config/environments/development.rb", "config.i18n.raise_on_missing_translations = true")
+        end
+      end
+    end
+  end
+end

test/generators/suspenders/development/environment_generator_test.rb

@@ -0,0 +1,36 @@
+require "test_helper"
+require "generators/suspenders/development/environment_generator"
+
+module Suspenders
+  module Generators
+    module Development
+      class EnvironmentGenerator::DefaultTest < Rails::Generators::TestCase
+        include Suspenders::TestHelpers
+
+        tests Suspenders::Generators::Development::EnvironmentGenerator
+        destination Rails.root
+        setup :prepare_destination
+        teardown :restore_destination
+
+        test "raise on missing translations in development" do
+          run_generator
+
+          assert_file app_root("config/environments/development.rb") do |file|
+            assert_match(
+              /^ +config.i18n.raise_on_missing_translations = true$/,
+              file
+            )
+          end
+        end
+
+        def prepare_destination
+          backup_file "config/environments/development.rb"
+        end
+
+        def restore_destination
+          restore_file "config/environments/development.rb"
+        end
+      end
+    end
+  end
+end