Allow utilizing existing commands without registering them. #2138

Description

With SlashCommandProcessor.RegisterCommands = false, we cannot utilize existing commands that have already been registered.
This is due to the fact SlashCommandProcessor.applicationCommandMapping only gets assigned within SlashCommandProcessor.RegisterSlashCommandsAsync which is conditionally invoked based on the SlashCommandProcessor.RegisterCommands property.

Proposal;
Still map previously registered commands that have been detected as "not changed" so they can still be used without sending any command registrations over the wire.
Not really sure if this should be user configurable or not, since this may be only a me thing.

Specify the libraries you want this feature request to affect

DSharpPlus.Commands

Other considerations

No response