CI: Add Ruby 4.0 in matrix (#68)

Testing with Ruby 4.0

Author: kojix2

.github/workflows/ci.yml

@@ -23,6 +23,7 @@ jobs:
         - macos-latest
         - windows-latest
         ruby:
+        - "4.0"
         - "3.4"
         - "3.3"
         - "3.2"