Migrate to gm alternative as the package has been sunset

[dsaltares]

https://github.com/aheckmann/gm has been sunset and archived. It'd be great if pdf2pic could move to an alternative as future vulnerabilities will not be patched -> #222.

Thanks!

[mskec]

I don't believe there are alternative packages that do the same thing.
An alternative would be to execute binaries using cross-spawn directly, in which case it might make more sense to fork gm and patch any security vulnerabilities in its dependencies.