This commit does major refactor of "Worker" and "WebWorker",
in order to decouple them from "ProgramState" and "Flags".
The main points of interest are "create_main_worker()" and
"create_web_worker_callback()" functions which are responsible
for creating "Worker" and "WebWorker" in CLI context.
As a result it is now possible to factor out common "runtime"
functionality into a separate crate.
This commit disables source mapping of errors
for standalone binaries. Since applying source
maps relies on using file fetcher infrastructure
it's not feasible to use it for standalone binaries
that are not supposed to use that infrastructure.
