ci: enable -Werror=dev

Turn developer & deprecation warnings into errors.

.github/ci-test-each-commit-exec.py

@@ -27,6 +27,7 @@ def main():
         "cmake",
         "-B",
         "build",
+        "-Werror=dev",
         "-DCMAKE_C_COMPILER=clang",
         "-DCMAKE_CXX_COMPILER=clang++",
         "-DWERROR=ON",