fix: type guard for sendable text-based channels

[vladfrangu]

Please describe the changes this PR makes and why it should be merged:

Adds in BaseChannel#isSendable, which is a mix of isTextBased() and checking if you can actually send messages in the channel. This way, we can have the correct types for interaction.channel, and you can assert whether you got the interaction from a Group DM Channel or not

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR changes the library's interface (methods or parameters added)

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[imnaiyar]

IG this (and the JSDoc) can also be updated to include PartialGroupDMChannel since it's considered text-based now https://github.com/discordjs/discord.js/blob/4594896b5404c6a34e07544951c59ff8f3657184/packages/discord.js/src/util/Constants.js#L96-L122