refactor: improve adding prefixed commands

[AstreaTSS]

What type of pull request is this?

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

Description

This just improves Client.add_prefixed_command and PrefixedCommand.add_command. There's a slight speedup and the error message is slightly better.

Changes

• Use commands.get instead of iterating through the dict's values to see if a command is a duplicate.
• There's been minor changes to the error message that makes it more grammatically correct. I also have no idea what Black did there but it did a thing--

Checklist

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