feat: hard multi-version protections

[guybedford]

Adds the hard version protections suggested in #484 - freezing the importShim object on the global and also freezing its properties.

I would still like to verify before landing this:

1. The shim mode switch is definitely necessary. I would have thought the ep markers would have been conflict-avoiding here therefore this would be unnecessary.
2. The defineValue(self, '_d', undefined) thing - what version this is deconflicting with and why.

[guybedford]

Another thing we could do here to help solve the race issue in future is to support a new init option, version, where when set, only that exact version of es-module-shims is permitted to register on the page. I've added an experimental commit to this effect, @frandiox would be interested to get your feedback on that as well.

[frandiox]

Thanks for tackling this!

The shim mode switch is definitely necessary. I would have thought the ep markers would have been conflict-avoiding here therefore this would be unnecessary.

This seems needed in some versions indeed. For example, 2.0.0 or 1.4.0 + Firefox

The defineValue(self, '_d', undefined) thing - what version this is deconflicting with and why.

I found about the _d here, which is code included in versions around 2.0.0 (removed in 2.0.3), including earlier versions like 1.10.
It's hard to reproduce but it happens in Safari with these versions sometimes. Changing the _d seemed to disable the checks in these versions. I'm not 100% sure about the mechanism, but I think that assigning undefined in _d propagates the undefined value to dynamicImport, and thanks to that it gets overwritten by this custom function 🤔 -- I'm not sure why the original u=>import(u) is problematic but it seems to create issues indeed (maybe it can't be rewritten by the first loaded polyfill? Unsure). It makes me suspect there might be some mixed behavior between the 2 loaded polyfills but I don't really know.

I've pushed a branch to my test repo with the patched polyfill, in case you want to test it: https://github.com/frandiox/importmap-polyfill-test/tree/fd-patch-test -- you can play with removing async attribute as well to ensure order.

Hope this is helpful!

[guybedford]

Releasing in 2.4.0 now.