Add rake task to dump graphql schema

$ rake graphql:dump

The task creates a JSON and IDL dump files.

Also generate the first dump files.

Rakefile

@@ -8,6 +8,9 @@ begin
   require 'spree/testing_support/extension_rake'
   require 'rubocop/rake_task'
   require 'rspec/core/rake_task'
+  require 'graphql/rake_task'
+
+  GraphQL::RakeTask.new(schema_name: 'Spree::Graphql::Schema')
 
   RSpec::Core::RakeTask.new(:spec)
 
@@ -31,6 +34,17 @@ task :test_app do
   Rake::Task['extension:test_app'].invoke
 end
 
+namespace :graphql do
+  desc 'Dumps GraphQL schema as IDL and JSON.'
+  task :dump do
+    Rake::Task[:first_run].invoke
+
+    require File.expand_path('spec/dummy/config/environment.rb', __dir__)
+
+    Rake::Task['graphql:schema:dump'].invoke
+  end
+end
+
 namespace :dev do
   desc 'Setup development app'
   task :setup do

schema.graphql

@@ -0,0 +1,2 @@
+type Query {
+}