Add workflow to run CI

.github/workflows/main.yml

@@ -0,0 +1,19 @@
+name: Main
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    env:
+      RACK_ENV: "test"
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ruby/setup-ruby@v1
+        with:
+          bundler-cache: true
+      - run: bundle exec rake spec