Testing double-fetching of `module`/`nomodule` JS code (including [the Safari hack](https://gist.github.com/samthor/64b114e4a4f539915a95b91ffd340acc))

```
<script type="module" src="module.js"></script>
<script nomodule src="nomodule.js"></script>
```

Test page: https://jg-testpage.github.io/es-modules/module-nomodule/

|IE/Edge|Firefox|Chrome|Safari |fetches module|fetches nomodule|executes|    |
|:-----:|:-----:|:----:|:-----:|:------------:|:--------------:|:------:|----|
|  15-  |  59-  |  55- |10.0-  |      v       |        v       |nomodule|❌  |
|  16   |       |      |10.1/3 |      v       |        v       |module  |❌  |
| 17-18 |       |      |       |  double!     |        v       |module  |❌❌|
|       |       |56-60 |       |              |        v       |nomodule|✅  |
|       |  60+  |  61+ |11.0+* |      v       |                |module  |✅  |

Summary:
- ✅ no browser does double execution (provided [the Safari hack](https://gist.github.com/samthor/64b114e4a4f539915a95b91ffd340acc))
- ✅ modern Chrome and Firefox never fetch more than necessary
- ⚠ Safari <11 may or may not double fetch (even with the hack); it does not on small test pages, but in real complex pages it does (it seems deterministic, but not clear what's the exact trigger)
- ⚠ Safari 11+ may still double fetch in some cases (see https://bugs.webkit.org/show_bug.cgi?id=194337)
- ❌ pre-2018 browsers do double fetches
- ❌❌ latest Edge does triple fetch (2x module + 1x nomodule)