feat: propagate external modules to swc Bundler

[manzt]

Towards #51

Collects any Source::External module objects from the ModuleGraph and propagrates them as external modules to the swc bundler. Previously, a graph with external modules would throw a runtime exception.

This shouldn't introduce any changes in behavior to the TypeScript API, since the loader since the JsLoader from createCache doesn't expose a way to mark external dependencies.

[CLAassistant]

All committers have signed the CLA.

[manzt]

In a follow up, trying to think of how to support Source::External that are resolved to via a custom graph resolve (e.g., something based on import-maps). Right now, I think for the following:

import * as foo from "foo";

export const bar = () => foo();

{
  "imports": { "foo": "https://example.com/foo.js" }
}

if "https://example.com/foo.js" were to be marked as Source::External in the graph, the Load impl for BundlerLoader would need to rewrite the "foo" import to the full resolved "https://example.com/foo.js" in order to stop swc from trying to resolve foo.

[dsherret]

LGTM. Thanks!