Add bot to thread when slash command is used

[advaith1]

In private threads, slash commands can be used, but if the bot is not in the thread, it will not be able to read/manage messages.

If the bot has access to the parent channel, it can reply to the interaction and ping itself, adding itself to the thread. However, this is annoying to handle and it would be nicer if Discord did it automatically, treating using a bot's command as mentioning the bot.

Alternatives possible currently (suboptimal ux):

• just break when the bot isn't in the thread
• respond and ask the user to ping the bot
• pinging itself to add itself to the thread, then processing the command

Alternatives Discord could do:

• Let devs set whether to add the bot as a field in the command
• Add an endpoint where the bot can add itself to the thread when there is an interaction (ex: /interactions/:id/:token/join-thread); cleaner than responding to the interaction and pinging itself