types(anychannel): add PartialGroupDMChannel

[ImRodry]

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

This fixes a bug with Channel#isDMBased() where it wouldn't correctly infer that a channel isn't a group DM

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

[kyranet]

Tests need to be updated according to CI's logs:

discord.js:test: typings/index.test-d.ts(951,51): error TS2345: Argument of type 'string | null' is not assignable to parameter of type 'undefined'.
discord.js:test:   Type 'null' is not assignable to type 'undefined'.

[ImRodry]

Yeah I know, my bad, I didn’t see the error when committing and haven’t had time to fix since. Will do later today