Add Ruby 4.0 to CI matrix for testing

Author: kojix2

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
       fail-fast: false
       matrix:
         os: ["ubuntu", "macos", "windows"]
-        ruby: ["3.4"]
+        ruby: ["3.4", "4.0"]
     runs-on: ${{ matrix.os }}-latest
     steps:
       - uses: actions/checkout@v6