Handle non standard skimmers (haven_labelled)

I had to add a lot of ugly checking code to get skimr to build and pass tests when haven is not installed.   This issue is to find a better way (or at least a simpler way such as having a standard function) to check for haven.  

Also we could switch to using `rlang::check_installed()` for doing the checks. 

Also we should be able to simplify some of the handling of fonts.

