Dynamic import does not work in all browsers

TLDR: Dynamic import does not work in all browsers, so usage of ES6 imports in workers to support pthreads is problematic

When targeting an `.mjs` output, emscripten will export JS as ES6. This mostly works, but FF does not yet support ES6 imports in workers, so I get this error in FF:

```
Error: Dynamic module import is disabled or not supported in this context
```
(from module.worker.js)

If this is a known limitation, perhaps it should be noted [here](https://emscripten.org/docs/tools_reference/emcc.html?highlight=emcc#:~:text=ES6%20JavaScript%20module).

For anyone wanting to support more than just Chrome this makes ".mjs" a non-starter when using pthreads. If pthreads workers can't be implemented without relying on ES6, perhaps emscripten could emit a big warning when this happens warning that these flags are currently incompatible with cross-browser support?

ref https://bugzilla.mozilla.org/show_bug.cgi?id=1247687