fix(openapi-typescript-helpers): keep index.d.cts in npm distribution (#1788)

* keep index.d.cts in npm distribution

* changeset

* format

Author: yukukotani

Diff for: .changeset/soft-roses-teach.md

@@ -0,0 +1,5 @@
+---
+"openapi-typescript-helpers": patch
+---
+
+keep index.d.cts in npm distribution

Diff for: packages/openapi-typescript-helpers/package.json

@@ -23,6 +23,7 @@
     },
     "./*": "./*"
   },
+  "files": ["index.js", "index.cjs", "index.d.ts", "index.d.cts"],
   "homepage": "https://openapi-ts.dev",
   "repository": {
     "type": "git",