chore: dev deps and version constraints, vscode launch cfg, purge cron error handling

[tigattack]

Initial Checklist

• Has @tigattack been added as a reviewer?
• If applicable, have the relevant project(s), milestone(s), and label(s) been applied?
• If applicable, have you added details of the cog to the readme as per README.md?

Details

Does this resolve an issue?
No

Changes

Features / Fixes

• Rename RedBot VSCode launch config
• Add missing development dependencies
• Add missing version constraints for cog dependencies
  • purge — croniter constrained to >=6.0.0 (current latest)
  • verify— python-Levenshtein constrained to >=0.27.1 (current latest)
• Improve accuracy of cron syntax error handling in purge cog by catching the more specific CroniterBadCronError exception instead of CroniterError.