Migrate from Commandant to Apple's open source Swift Argument Processor.

Commandant has been dormant for 4+ years

Swift Argument Parser supports subcommand aliases (#281), can generate bash, zsh & fish completion scripts (#453 & #222), provides argument validation & error messages (#480 & #533), is more concise/clean, and is actively being developed with Apple's full commitment.

I have a working branch ready to be submitted as a PR once my PR with the fix for the current main lint issue has been merged.

This change:

• updates the minimum Swift Tools version from 5.3 to 5.5 (supported by macOS 11 & later; 11 & 12 are already EOL, so this shouldn't cause issues)
• moves everything from MasKit to mas (I also renamed MasKitTests to masTests for consistency)
• removes the dependency on the swift-format library, as it depends upon an older version of Swift Argument Parser. Brewfile now contains brew swift-format. lint & format now call swift-format as a normal command.