Introduce CommandArgument and CommandArguments for slash commands

[sieadev]

This PR adds two new classes to improve slash command argument handling:

• CommandArgument – represents a single argument with type-safe access methods (e.g., .getAsString()).
• CommandArguments – a container for all arguments passed to a command, replacing the old Map<String, Object> approach.

Changes:

• Slash command methods now support only two valid signatures:
  • (SlashCommandInteractionEvent event)
  • (SlashCommandInteractionEvent event, CommandArguments args)
• Methods using the old (event, Map) signature are marked as deprecated at compile-time.
• Unsupported method signatures now produce compile-time errors.
• Existing modules using the old (event, Map) signature continue to work but receive deprecation warnings.