Add workflow to ensure the gem installs fine on truffleruby

Author: eregon

.github/workflows/truffleruby.yml

@@ -0,0 +1,20 @@
+name: TruffleRuby
+
+on: [push, pull_request]
+
+jobs:
+  truffleruby:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: truffleruby-head
+        bundler-cache: true
+    - name: Test installing the gem on TruffleRuby
+      run: |
+        bundle exec rake compile
+        bundle exec rake build
+        gem install pkg/stringio-*.gem
+        ruby -rstringio -e 'p StringIO'