feat: support loading extensions without an entrypoint

[LordOfPolls]

What type of pull request is this?

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

Description

Allows naff to handle loading an extension file without an entrypoint. Note that it's impossible to determine intent, this just loads anything that bases Extension in a file - I still strongly advise declaring an entrypoint

Changes

• Update load_extension to parse a module for Extension objects if no setupfile is defined

Checklist

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