Serenity Slash Decode

After banging my head against a wall trying to get slash commands to make sense in Serenity, in addition to the impending Discord intent changes, I decided to make a library to help with easier parsing of command arguments. Inspired by Clap's ArgMatches, you're able to parse arguments in one call, match which function to execute for which interaction, and extract values from the arguments. You're even able to use ?s to exit early from your command's individual function.