Add htmlrewriter to default server externals (#80819)
This package needs to be externalized to have it's wasm dependencies
traced properly for node runtime so this adds it to our default list.
x-ref: [slack
thread](https://vercel.slack.com/archives/C07LQPFERGF/p1750713377659779?thread_ts=1750693981.162239&cid=C07LQPFERGF)
