Optional combine_pdf + support yarn pnp

[le0pard]

Reason for this changes

Gem combine_pdf needed only for middleware and only for front_cover and/or back_cover functionality. This mean, that people which use grover directly by display_url or raw html need to have this gem as additional dependency. Problem, that combine_pdf contain as deps https://github.com/caiges/Ruby-RC4 , which is not supported any more (dead dependency).

https://github.com/boazsegev/combine_pdf?tab=readme-ov-file#unmaintained---help-wanted - combine_pdf also unmaintained

Maintained solutions to use pdfunite cli from poppler library or to use https://hexapdf.gettalong.org/examples/merging.html (paid solution)

By this proposal I making combine_pdf as optional and not install as dependency, so developers, which need middleware and front_cover and/or back_cover functionality can install it separately in application.

Also added ability to override js runtime bin, so in this case devs, which use non standard env, like yarn pnp, can override hardcoded node bin (example added in README). Possible fix for #262

[abrom]

Nice one @le0pard 👍

I've got a few suggestions on some copy improvements, but otherwise looking solid

[le0pard]

Thanks @abrom , fixed

[abrom]

Fantastic, thanks @le0pard 👍

[abrom]

Released in v1.2.0