Update: Add Extera Next client

[awtj8o81ryywg793]

Extera Next is the successor of the now obsolete Extera. However, it isn't currently listed on matrix.org. I'm not the maintainer of, nor a contributor to, the client, so I'm unaware of what features it currently supports. Since it's a fork of Fluffychat, I assume it supports what that currently supports. Would that be correct, @officialdakari?

[officialdakari]

Hello! Yes, Extera Next supports everything Fluffychat does, and it also supports threads.