Complement !mdn with /mdn

[vcarl]

This lets autocomplete function as a search field, so folks can make sure they're sending what they mean to

[what-the-diff]

PR Summary

• Added New /mdn Command
  This update introduces a new command, /mdn, that users can use to search for resources from the Mozilla Developer Network (MDN). This will enhance user capability for resource discovery, including the added benefit of auto-complete for search queries.

• Modified Command List Message
  The command list message, displayed when interacting with the bot, has been updated in src/features/commands.ts to indicate the presence of the new /mdn command. This ensures users are aware of its addition to streamline their searches for resources.

• Registered the New Command
  The src/index.ts file has been updated in order to register the new /mdn command. This means the bot is now cognizant of the new command, allowing users to fully utilize the search capability provided by this update.