WhisperKit CLI cleanup

[jkrukowski]

Before taking on https://github.com/argmaxinc/WhisperKit/issues/36 I decided to do a little cleanup in the CLI

• moved command line arguments to a separate WhisperKitArguments struct
• extracted a separate transcribe subcommand
• changed executable name to whisper-kit

so rn in order to run the transcribe command the user has to type swift run whisper-kit transcribe It's going to be convenient if we're going to add more subcommands in future (e.g. swift run whisper-kit translate)