Core: IE11 workaround for currentScript

[RunDevelopment]

This fixes #2102.

The IE11 workaround was inspired by this.

[BrainStone]

Are you sure #2103 needs fixing and not #2102? ;)

[RunDevelopment]

Thanks for pointing that out 😅

[muenzpraeger]

Hey - I wanted to use Prism embedded into a web component, but ran into the issue that this IE11 hack is used. Specifically it is that in my environment scripts[i] can be undefined.

Any thoughts on accepting a PR that extends the if check on undefined? Happy to put that in.

[RunDevelopment]

@muenzpraeger How can scripts[i] be undefined? scripts is a HTMLCollection, so iterating with for in should iterate over all indexes.

[muenzpraeger]

It runs within a secure context of that environment.

[RunDevelopment]

Ok, after a dive into the documentation, I found the issue. The "global" document object is a mock object that is an instance of the SecureDocument interface (class?). This explains why the currentScript property is absent (it's listed as "not supported") but it doesn't explain why getElementsByTagName doesn't return an HTMLCollection since the function is listed as supported since v46.

My conclusion is that this is a bug with SecureDocument. The getElementsByTagName function is supposed to return a value that fulfills the HTMLCollection interface but it doesn't. Please report this bug, so the authors of SecureDocument can fix it.