Issue description

When using Shakapacker >= 8, calling WickedPdfHelper#running_in_development? raises an exception:

uninitialized constant WickedPdf::WickedPdfHelper::Assets::Webpacker (NameError)

        if Webpacker.respond_to?(:dev_server)
           ^^^^^^^^^

Expected or desired behavior

Calling WickedPdfHelper#running_in_development? returns a boolean, without raising an exception.

Causes

This error occurs because Shakapacker 8 renamed all Webpacker constants to Shakapacker.

System specifications

wicked_pdf gem version (output of cat Gemfile.lock | grep wicked_pdf): 2.8.2

shakapacker gem version : 8.0.2