Export thread and its messages

### Your use case

#### What would you like to do?

I would like to be able to export a thread and its messages without needing to export all messages that are not part of the thread.

#### Why would you like to do it?

The company I work at like to keep the main chat free of clutter and therefore when replying it is done in a thread, that thread sometime gets deleted or lost so when looking back at a discussion/reply chain it would be easier for the one person that has the intact thread to be able to document it in our wiki tool.

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

By having a option like `Export chat` but in the thread interface in a three dot style action menu with the text `Export thread`.

### Have you considered any alternatives?

Exporting all messages from the chat and then manually removing the messages that are not necessary.

### Additional context

> Exporting all messages from the chat and then manually removing the messages that are not necessary.

Since threaded messages are not "attached" to the main message the threaded messages seem to be added at a later date and with a ref ID to the main message.

Also, don't know if this enhancement would require a spec change in the matrix spec

Start by locating the existing `Export chat` action and the three-dot action menu in the thread interface. Check how threaded messages are represented and whether exporting only a thread requires a Matrix spec change; done means the thread export contains the thread and its messages without unrelated room messages.

Written by the indexing model from the issue text.