Add support for Discord

[instantiator]

Context

It's time to add Discord to the family of supported networks.

open-social-distributor used Discord.Net, which may be a good first library to explore.

Blocked by: #19

[github-actions]

Status: Ready to merge ✔️

Issues blocking this PR:

• Support multiple identities on the same social network #19 ✔️

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This comment was automatically written by the Blocking Issues bot, and this PR will be monitored for further progress.