chore: update build script and remove unused Vercel configuration

- Modified the build script in package.json to rename the output file from server.js to index.js.
- Removed the vercel.json configuration file as it is no longer needed.
- Updated ESLint configuration to reflect changes in file references.

vercel.json

@@ -1,9 +0,0 @@
-{
-    "$schema": "https://openapi.vercel.sh/vercel.json",
-    "framework": "hono",
-    "buildCommand": "pnpm run build:vercel",
-    "outputDirectory": "dist",
-    "github": {
-        "silent": true
-    }
-}