Finer-grained chat log export control

### Your use case

#### What would you like to do?

Export a particular slice of the timeline, starting and ending with some particular messages, without having to eyeball the number of messages.

#### How would you like to achieve it?

Ideally, we'd be able to select a timeline slice by clicking and dragging, perhaps after entering some kind of special mode. The action bar could then offer to export that slice. This would also be useful for copying a block of messages into the clipboard (so might be related to https://github.com/vector-im/element-web/issues/893).

### Have you considered any alternatives?

Barring the above, having a way to export *starting* from some message would also be better than the current situation.

### Additional context

_No response_