Add support for migrating config from the Discord Rare Drop Notifier plugin

[pajlada]

After discussions with the current maintainer of the plugin @BossHuso we'd like to add the ability to migrate the user's plugin config into Dink.

I see this being done with a user command (e.g. ::dinkmigrate). Name might need to be more specific, or it accepts a parameter, or it popups up a dialog with user choice or something.

Once this is implemented on our end, the Discord Rare Drop Notifier plugin can prompt users to use the command to migrate their config.

Open questions: Should we remove the config keys from the Discord Rare Drop Notifier plugins after migrating them?

Plugin repo: https://github.com/BossHuso/discord-rare-drop-notificater