Isolate the Member and User classes

[NeloBlivion]

pycord v2 treats Member as an extended User object and copies its attributes; while this isn't entirely wrong, Discord treats Member and User as unique objects. In many cases a Member payload includes a user key, but this is not guaranteed.

• next should completely separate Member and User as per their respective specs.
• Where there is an inner user key, it should use a new MemberWithUser class (name as you wish) that's functionally similar to v2's Member.
  • MemberWithUser inherits both Member and User, overriding Member properties over matching User properties.
• Member can resolve to MemberWithUser if there's a cached User available on the client.

This change is potentially awkward for users, but it is worth getting a better match with the API and properly knowing what attributes to expect. You are free