fix: allow coroutines for prefixed commands

[AstreaTSS]

What type of pull request is this?

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

Description

While coroutine functions were always intended to work with prefixed commands if you used them in an annotation, it turns out the support to them was broken. This fixes that.

As a fun side effect, this also may fix a couple of other cases where functions weren't being properly parsed.

Changes

• Use isroutine instead of isfunction to ensure the function is caught in all cases.
• If the function has 2 parameters, return the function instead of unnecessarily wrapping it (which also blocks inspect.iscoroutinefunction from doing its job).
• If the function has 1 parameter, use an asynchronous function wrapper instead of a lambda, using maybe_coroutine for the actual function itself.

Checklist

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