Implement /sort_desc Command

[tanamaroby]

Issue Title: Implement /sort_desc Command

Issue Description: As a user of our Discord bot, I want to be able to use a "/sort_desc" command to sort a list of integers in descending order, so that I can easily organize numerical data within Discord.

Feature Description:

• The "/sort_desc" command should take one or more arguments, representing the list of integers to be sorted.
• The bot should validate each argument to ensure it is a valid integer. If an argument is not a valid integer, the bot should respond with the message "Invalid argument: only digits!".
• If all arguments are valid integers, the bot should sort them in descending order and respond with the sorted list.
• The bot's response should include the original list of integers and the sorted list.

Acceptance Criteria:

• The "/sort_desc" command is successfully implemented and tested.
• Invoking the "/sort_desc" command with one or more arguments should result in the bot sorting the list of integers in descending order and responding with the sorted list.
• If any argument is not a valid integer, the bot should respond with the message "Invalid argument: only digits!".
• The bot's response should include the original list of integers and the sorted list.