keep index.d.cts in npm distribution

Author: yukukotani

packages/openapi-typescript-helpers/package.json

@@ -23,6 +23,12 @@
     },
     "./*": "./*"
   },
+  "files": [
+    "index.js",
+    "index.cjs",
+    "index.d.ts",
+    "index.d.cts"
+  ],
   "homepage": "https://openapi-ts.dev",
   "repository": {
     "type": "git",