prep work for more commits

[heckj]

Small pile of misc things here:

• updated dependencies to the latest swift-png (4.4) and related deprecation changes
• updated CI to use setup-swift to simplify version of swift
• I made a change in my fork of the repo to work from branch main over master - CI files updated to work from main, although I'm not sure if you'd want to do the same
• changes references to the updated GitHub repository owner, and your name
• added a .mailmap file with prior email address (replicated from swift-png) to handle prior commits/contributor details

[tayloraswift]

thank you!

[heckj]

For the CI changes in particular, you'll likely want to change the default branch for your repo to main. If you'd prefer to stick with master, I can revert that out for you and toss up another PR. I changed mine to main mostly because the older naming convention just feels wrong to me now.