Update rapidcheck upstream, add pkg-config, add uscan watch file

This change updates to the newest upstream (ff6af6fc683159deb51c543b065eba14dfcf329b) from Dec 2023. This upstream release produces pkg-config dev files.

At the same time, I added a debian/watch file. This will allow downloading the current upstream tarball, and should allow debian to watch for newer changes in upstream.

Unfortunately, I don't think the current rapidcheck release version format can be supported by debian/watch. So, while the previous release version was 0~1048-a5724ea-1, the new one in this merge-request is 0.0~git20231214.ff6af6f-1.

The debian/watch file and version format are not that important, but the upgraded upstream and enabling share library building seem like good ideas for the package.

I tested these with the nix package, which uses rapidcheck at build time, and the latest upstream nix release wants to use pkg-config for rapidcheck.