[Pusher] Display "Someone is writing" in a discussion

## Feature Request
It would be nice if Pusher would display that someone is currently writing a post in a discussion. This is very useful in hot discussions. Then users know they can wait with their post until someone else posts.
We have just migrated our huge forum to Flarum and the lack of this feature is annoying.

**Describe the solution you'd like**
A detailed description of your proposed solution. Include:
- When someone is just writing in a discussion, Pusher could display information to other users below the last post: "Someone is currently writing".

Start by locating the Pusher integration and the discussion post view mentioned in the request. Check how live discussion updates are handled and determine what event should represent someone writing. Done means other users see “Someone is currently writing” below the last post while a participant is composing.

Written by the indexing model from the issue text.