fix: add Vercel config for monorepo build

Author: junhoyeo

vercel.json

@@ -0,0 +1,6 @@
+{
+	"buildCommand": "pnpm --filter use-color build && pnpm --filter @use-color/playground build",
+	"outputDirectory": "packages/playground/.next",
+	"installCommand": "pnpm install",
+	"framework": "nextjs"
+}