The initial implementation of `importScripts()` in #11338 used
`reqwest`'s default client to fetch HTTP scripts, which meant it would
not use certificates or other fetching configuration passed by command
line flags. This change fixes it.
Classic worker scripts are now executed in the context of a Tokio
runtime. This does mean we can not spawn more tokio runtimes in
"op_worker_sync_fetch". We instead spawn a new thread there, that can
create a new Tokio runtime that we can use to block the worker thread.
This commit implements classic workers, but only when the `--enable-testing-features-do-not-use` flag is provided. This change is not user facing. Classic workers are used extensively in WPT tests. The classic workers do not support loading from disk, and do not support TypeScript.
Co-authored-by: Luca Casonato <hello@lcas.dev>
