Allow commands to be written using plaintext as well as slash commands

[stelitop]

Currently, commands are only executed as standard slash commands. It would be convenient to allow for text messages using a command prefix and a command name to be parsed and linked to the same code for a slash command.

This behaviour is not simple to resolve. Text messages do not have to reply to ChatInputInteractionEvents, so if the method also works as a SlashCommand they cannot reply to the event directly. The EventResponse alternative can handle this problem.

Another problem would be the use of a "command prefix" often seen, such as !, $ etc. This prefix should be customisable on a per-guild basis, as it can conflict with the prefixes of other bots or the users might not like the default prefix.