Add polyfill for "promise.withResolvers"

[fancycode]

Hmm, I will have to check if we can detect errors like this. pdf.js itself could be loaded (we already check this) but then failed to execute some code internally later.

But we could add a polyfill for Promise.withResolvers in any case. Thanks for checking.

Originally posted by @fancycode in #363 (comment)