Fixes issue #466

[afbarbaro]

The fix is conceptually really simple: use a dynamic import (or require under cjs) to import MessageChannel from node's worker_threads module if we detect that the browser's instance of MessageChannel is undefined.

Nevertheless, this turned out to be more complicated that it should due to the lack of widespread support for TLA (top-level await). So, unfortunately, the fix is only available for the umd bundle, otherwise the esm bundle would just work on node and would break when running on browsers.

I've put comments on the code explaining that when TLA are widely available this can be fix for both module systems.

Overall, I'm not super thrilled with how this turned out. Open to feedback and ideas on how to improve it.

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[afbarbaro]

@googlebot I signed it!

[eliellis]

Would love to see this land!

[benjamind]

I'm beginning to do a scrub of PRs for merge candidates for the next version. State of feature support for Top Level Await has changed since this PR was initially posted, would someone be willing to take a second look at it now that we have TLA in node and all browsers?