Add a method for fetching the children of a CategoryChannel

[abitofevrything]

Description

Adds a method to fetch the children of a CategoryChannel from the API or from the cache. Also adds an alternative to Guild.fetchChannels that operates only on the cache.
sure to link issue/PR and describe said problem

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• Ran dart analyze or make analyze and fixed all issues
• Ran dart format --set-exit-if-changed -l 160 ./lib or make format and fixed all issues
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works