Add rbs validate to CI

Author: znz

Diff for: .github/workflows/test.yml

@@ -34,3 +34,6 @@ jobs:
         bundler-cache: true # 'bundle install' and cache
     - name: Run test
       run: bundle exec rake test
+    - name: RBS validate
+      run: bundle exec rbs -r openssl -r digest -r uri -r erb -r singleton -r tempfile -r socket -I sig validate
+      if: ${{ ! startsWith(matrix.ruby, '2.') }} # rbs requires ruby 3.0+

Diff for: Gemfile

@@ -5,3 +5,6 @@ gemspec
 gem "rake"
 gem "test-unit"
 gem "test-unit-ruby-core"
+
+# rbs requires ruby 3.0+
+gem "rbs", require: false if !RUBY_VERSION.start_with?('2.')