Add ability to move items in a collection

[JonnyWong16]

Description

Adds the Collection.moveItem() method to move items in a collection (analogous to playlists). This allows for custom collection ordering exposed in Plex Web 4.60.1.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable