feat: use getopt to parse command-line arguments

[cybardev]

Description

Given that the complexity of this script has been rising recently, we think it's big enough to warrant using getopt to parse command-line arguments.

Changes Requested

• use getopt to parse command-line arguments
• ~~try to make certain combinations invalid, e.g. cutefetch --random eyes paired with an eye number. (randomizer to be implemented for #9)~~

Acceptance Criteria

• script uses getopt to parse command-line arguments
• ~~rejects invalid flag combinations~~

Resources

• getopt tutorial: https://www.baeldung.com/linux/bash-parse-command-line-arguments

Blockers

• Blocks: #9
• Blocked-by: N/A

[cybardev]

UPDATE: flag validation will be done in a separate ticket. For now, the goal is just to have feature parity with the current script but using getopt