Commands to move read receipt and/or fully read marker to point to a particular event

### Your use case

#### What would you like to do?
`/readreceipt ` (alias: `/ack `) : set the read receipt to `event-id`
`/threadreadreceipt ` (alias: `/tack `) : set the thread read receipt to `event-id` in the thread started by `thread-relation-id`
`/fullyread `: set the fully read marker to `event-id`
`/ackallthreads` : set the read receipts and fully read markers of the room and all threads to the latest known events
When used without arguments, a pop-up with the current marker positions should be returned. (`/ack` without arguments should instead point the read receipt to the latest event possible)
#### How would you like to achieve it?
`/readreceipt` should also check for the forward-only invariant client-side, before sending the request to the server side.

### Have you considered any alternatives?

"Mark as unread" but that would be coarse.