This commit:
- removes "fmt_errors::PrettyJsError" in favor of "format_js_error" fn
- removes "deno_core::JsError::create" and
"deno_core::RuntimeOptions::js_error_create_fn"
- adds new option to "deno_runtime::ops::worker_host::init"
Due to a bug in V8, terminating an isolate while a module with top-level
await is being evaluated would crash the process. This change makes it
so calling `worker.terminate()` will signal the worker to terminate at
the next iteration of the event loop, and it schedules a proper
termination of the worker's isolate after 2 seconds.
