Add SPM Support

[msmollin]

Hello! I'm in the process of migrating my project to SPM and SZTextView is honestly pretty close to being SPM compatible already in its current form, so I just shuffled some things around a bit to make SPM happy and added a Package.swift.

This is heavily informed from Nick Lockwood's GZIP project package file as the two projects are very similar in structure. The only difference is for some reason GZIP doesn't throw a warning about not excluding its Info.plist whereas this project does (when pulled into a downstream project via SPM). I'm not a SPM expert but I'm guessing it's something to do with the tools version, but that is entirely a guess.

Let me know if anything needs changing / updating!

[glaszig]

thanks for the contribution. i'm currently not working with swift/xcode so let me know when you think this pr is ready and i'll merge.

[msmollin]

@glaszig somehow missed your reply years ago 🤦 - been using this just fine. Feel free to merge.

[vijaytholpadi]

@glaszig Would it be possible to merge this please? A lot of companies are finally moving to SPM and this would be helpful. Thanks.

[glaszig]

thanks for beating me to it ;)