Convert test/multistream.js to TypeScript

Co-authored-by: moufmouf <1290952+moufmouf@users.noreply.github.com>

Author: Copilot

package.json

@@ -38,6 +38,7 @@
     "string-to-stream": "^3.0.1",
     "thunky": "^1.1.0",
     "typescript": "^5.9.3",
+    "vite": "^7.2.7",
     "vitest": "^4.0.15",
     "ws": "^8.18.0"
   },