Promise id is now created in core and passed back to JS.

Adds ModuleSpecifier, which wraps a URL. This is now passed around instead of
specifier and resolver strings.

It's unnecessary indirection and is preventing the ability to easily
pass isolate references into the dispatch and dyn_import closures.

Note: this changes how StartupData::Script is executed. It's no longer done
during Isolate::new() but rather lazily on first poll or execution.

This is just encoding my actual workflow; not suggesting that this
workflow is ideal. Previously I would edit sync_third_party.py each time
I ran it.

This fix is no longer necessary as the underlying V8 bug has been
fixed upstream.

This reverts commit 48bcfce0.

And improve bundle docs

Don't mix every http request in with the tests output.
Don't print that the file servers are starting unless
-vv flag is passed.

Capture the output of run with run_output which returns
stdout, stderr and exit_code. Test against this rather
than relying on sys.exit.

This patch makes it so that RecursiveLoad doesn't own the Isolate, so
Worker::execute_mod_async does not consume itself.

Previously Worker implemented Loader, but now ThreadSafeState does.

This is necessary preparation work for dynamic import (#1789) and import
maps (#1921)