feat: add set files util for pasteboard binding

[ClSlaid]

Hello, I'd like to implement file pasting on OSX, but sadly I know nothing about Swift and Obj-C. Thankfully, I found your project, and I'd like to add a patch to satisfy my need.

This PR merely adds a set_files method to Pasteboard. And it's expected to convert paths you put in to NSURLs and write into the clipboard.

~~Note that this PR is currently UNTESTED.~~