Complete the /collections related endpoints

[G10xy]

Hello maintainers, I have added new functionality to complete every /collections related API.

Changes made:

• Added a new method reorderCollection in the RedditSubreddit class
• Created a dedicated request class about adding to/removing from post to collection
• Centralized error handling with a static method

Please review the changes and let me know if you need any further information or modifications. Thank you for considering this contribution!