Regression: mainScriptUrlOrBlob ignored when USE_ES6_IMPORT_META is off

[JoeOsborn]

Per #23171 (comment)

The recent removal of USE_ES6_IMPORT_META broke a use case like this:

1. I want to host an emscripten-compiled package somewhere (e.g., a CDN) that is compiled to use pthreads
2. I want to instantiate a module from those scripts on another origin domain
3. Browsers refuse to download cross-origin workers, so I can't create threads.

This used to be supported using either an importScripts() hack in the worker or by downloading the whole script as a blob, but now that's not possible since the fix in #23171 forces the use of a specific file name.

[JoeOsborn]

For a bit more detail: Using a blob as the actual script means that import.meta.url is a blob, so new URL(...) can't work. Using a mainScriptBlobOrUrl is just ignored. So both of the tricks I've used in the past for cross origin workers are not available.

[sbc100]

Thanks Joe! We are obviously lacking some test coverage here.

[JoeOsborn]

I think a reasonable fix for this is to:

1. If mainScriptUrlOrBlob is set, create the worker as in the old non-use-import-meta case (or else deprecate that field under export_es6 and assert that mainScriptUrlOrBlob is not set)
2. If import.meta.url is a blob url, use it instead of creating a new URL.