Add suspenders:development_environment generator

To do:

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

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 +1,13 @@
+module Suspenders
+  module Generators
+    class DevelopmentEnvironmentGenerator < Rails::Generators::Base
+      def raise_on_missing_assets_in_test
+        environment "config.assets.raise_runtime_errors = true", env: "test"
+      end
+
+      def raise_on_missing_assets_in_development
+        environment "config.assets.raise_runtime_errors = true", env: "development"
+      end
+    end
+  end
+end

test/generators/suspenders/development_environment_test.rb

@@ -0,0 +1,47 @@
+require "test_helper"
+require "generators/suspenders/development_environment_generator"
+
+module Suspenders
+  module Generators
+    class DevelopmentEnvironmentGenerator::DefaultTest < Rails::Generators::TestCase
+      include Suspenders::TestHelpers
+
+      tests Suspenders::Generators::DevelopmentEnvironmentGenerator
+      destination Rails.root
+      setup :prepare_destination
+      teardown :restore_destination
+
+      test "raise on missing assets in tests" do
+        run_generator
+
+        assert_file app_root("config/environments/test.rb") do |file|
+          assert_match(
+            /^ +config.assets.raise_runtime_errors = true$/,
+            file
+          )
+        end
+      end
+
+      test "raise on missing assets in development" do
+        run_generator
+
+        assert_file app_root("config/environments/development.rb") do |file|
+          assert_match(
+            /^ +config.assets.raise_runtime_errors = true$/,
+            file
+          )
+        end
+      end
+
+      def prepare_destination
+        backup_file "config/environments/test.rb"
+        backup_file "config/environments/development.rb"
+      end
+
+      def restore_destination
+        restore_file "config/environments/test.rb"
+        restore_file "config/environments/development.rb"
+      end
+    end
+  end
+end