Given that we're already using the `external/dist/` folder for things we simply want to copy to pdfjs-dist during building, this patch *should* hopefully work since it's based on the suggestion in issue 12827.
As long as this only requires a *single/small* file, to fix the TypeScript definitions in es5-builds, this solution seem acceptable as far as I'm concerned. (Although, please note that I don't know enough about TypeScript to actually test the patch.)
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