fix: Account for function definition of command groups

[zevaryx]

What type of pull request is this?

• Non-breaking code change
• Breaking code change
• Documentation change/addition
• Tests change

Description

An oversight in Client.interaction_tree and Extension.interaction_tree led to errors during extension registration.

Example of breakage:

@slash_command(name="base", description="I break you")
async def base(self, ctx: InteractionContext):
    ...

@base.slash_command(sub_cmd_name="sub", description="I was broken")
async def sub(self, ctx: InteractionContext):
    await ctx.send("I will never even load, let alone run")

Changes

• Account for function definition of command groups to avoid errors

Checklist

• I've formatted my code with Black
• I've ensured my code works on Python 3.10.x
• I've tested my code