Create PurpleIRCv3Capabilities for managing capabilities for each connection

This implements everything in capabilities version 302 except for handling the
NEW and DEL commands from the server. I'm not sure how we can test that yet,
so we're ignoring it for now.

After the server has told us about all of the capabilities it supports, we
explicitly request cap-notify which we implictly request when we send
CAP LS 302. This is mostly to test our code and might be removed in the future.

Testing Done:
Connected to my local ergo and used ngrep to verify that everything was being sent correctly.

Reviewed at https://reviews.imfreedom.org/r/2100/