Add a `convert_to_cleaned_text_or_reply` util

[TizzySaurus]

As per python-discord/sir-lancebot#1074, we should add a CleanedTextOrReply converter which will allow an argument to be provided either by text, or by replying to a message (if an invocation has both then use the text).

There's currently no file for converters, so we'd need a new file -- either botcore/converters.py as per bot structure or botcore/utils/converters.py as per sir-lancebot structure.

I'd like to implement this myself.

[TizzySaurus]

Upon further discussion in dev-contrib, Zig and I have realised this will actually need to be a util function and not a converter, as converters require an argument to passed which won't always happen here.