Generate .d.cts file by copying over .d.ts

This is supposed to work according to `npx publint`:
> 1. pkg.exports["."].require types is not exported. Consider adding
> pkg.exports["."].require.types to be compatible with TypeScript's
> "moduleResolution": "bundler" compiler option. Note that you cannot
> use "./dist/index.d.ts" because it has a mismatching format. Instead,
> you can duplicate the file and use the .cts extension,
> e.g. pkg.exports["."].require.types: "./dist/index.d.cts"

packages/livekit-server-sdk/package.json

@@ -23,7 +23,7 @@
     "src"
   ],
   "scripts": {
-    "build": "tsup --onSuccess \"tsc --declaration --emitDeclarationOnly\"",
+    "build": "tsup --onSuccess \"tsc --declaration --emitDeclarationOnly && cp ./dist/index.d.ts ./dist/index.d.cts\"",
     "build:watch": "tsc --watch",
     "build-docs": "typedoc",
     "changeset": "changeset",