The "pdfjs-get.js" script downloads and unpacks "pdfjs-dist" in
"js/pdfjs" so it can be loaded from the PDF viewer. However, the PDF.js
version to download was got from "package.json", which just specifies
the desired ranges of versions to use, but not the exact versions
actually being built.

Due to that it could happen, for example after running "npm audit fix",
that "pdfjs-get.js" downloaded version X.Y, as it was the minimum
desired version specified in "package.json", but the dependencies were
resolved and built against version X.Z, as it was the actual version
specified in "package-lock.json".

To solve that now the PDF.js version is got from "package-lock.json"
instead, which provides the exact version to use.

Note, however, that this is just a "safety net" to avoid unexpected
mismatches as a result of running "npm audit fix" or similar; in general
the PDF.js version is expected to be in sync between "package.json" and
"package-lock.json".

Signed-off-by: Daniel Calviño Sánchez <[email protected]>