Add validation task to build gem

Preparation part for GH-544

.github/workflows/ci.yml

@@ -51,3 +51,4 @@ jobs:
           bundler-cache: false # runs 'bundle install' and caches installed gems automatically
       - run: bundle install
       - run: bundle exec rake test_all --suppress-backtrace='\/bundle'
+      - run: bundle exec rake validate_gem

Rakefile

@@ -12,7 +12,7 @@ else
   RuboCop::RakeTask.new
 end
 
-multitask(default: %i[test signature_all rubocop dprint])
+multitask(default: %i[test signature_all rubocop validate_gem dprint])
 
 desc('Keep light weight!')
 task(test: :test_core)
@@ -110,6 +110,10 @@ task(:generate_snapshots) do
   ruby('./scripts/generate_snapshots.rb')
 end
 
+task(:validate_gem) do
+  sh('gem build --strict --norc --backtrace ruby-ulid.gemspec')
+end
+
 desc('To prevent #69 anymore!')
 task(:view_packaging_files) do
   remove_entry_secure('./pkg')