FR: cpp_register() could allow non-.cpp extensions

[MichaelChirico]

Here, the resulting cpp11 file is hard-coded to have extension .cpp:

https://github.com/r-lib/cpp11/blob/df6d0eae400733efec1ebd92b5cf72f0a6c90264/R/register.R#L41

But sometimes we may need to write .cc extensions (e.g. if the other source files are also written as .cc).

Is there any reason not to offer an extra argument (extension = ".cpp") to allow this customization?

Happy as ever to file a PR :)

---

FWIW, on GitHub I see ~160M files with extension .cpp, vs. ~50M for .cc

[romainfrancois]

No particular reason, I'll review a PR :)