Add support for installing via Swift Package Manager

[NickEntin]

It would be great to add support for installing fishhook via Swift Package Manager. SPM has support for C language targets, although it recommends a specific file structure. If you'd like to keep the file structure unchanged, I think it should be possible to define things manually. I don't have much experience with SPM, so I'd need to do some more research as to exactly what options are available.

For background, AccessibilitySnapshot uses fishhook as a dependency. In order to add SPM support there, we need support in fishhook first.

Let me know if you're open to this. Thanks!