feat: add create_transaction_category

[CalvinChanCan]

This PR does the following adds a new method create_transaction_category which creates new transaction categories

Notes:

• Set default parameters for create_transaction_category based on the UI defaults
• When entering non-emoji strings (i.e. "test"), a new transaction category would be created with the icon string "test")

Appreciate any feedback. Thanks!

[hammem]

Thanks, @CalvinChanCan ! One quick question in there about how to represent emojis. I wasn't sure what Monarch's API truly supports to reduce ambiguity.

Thanks for the bugfix as well. If you can, put those in a separate PR from features and I'll be faster about review to get those out to folks.

[hammem]

Awesome, thanks!