Export PathFilter

[vergenzt]

Hi! The README mentions that PathFilter "is available", but it didn't appear to be exported. 🙂 PathFilter would be a very useful utility to have access to in my particular use case.

[vergenzt]

🤔 Build failure doesn't appear to be related to my change.

Also I just realized I can already import PathFilter via require('scandal/src/path-filter') -- so this isn't a blocker for me, just a nice-to-have. (Would rather not depend on something that's not explicitly exported by the package. 😬)

Edit: require('scandal/src/path-filter') wasn't working, which led me to realize I have to require('scandal/lib/path-filter) (the compiled JS module) for the unexported require to work.