When a user presses reply, add their message to the thread.

### Your use case

#### What would you like to do?

What the title says. Whenever a user writes a reply to a message, the reply should appear not only on the room's main timeline but also inside whatever thread starts with the message that's being replied to.

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

Because the current system forces the user to choose an audience: they can press reply and get the attention of the people reading the main TL, _or_ press thread and get the attention of the people reading the thread; which creates anxiety, as noted [here](https://github.com/vector-im/element-web/issues/19660#issuecomment-963423135) by @ara4n:

> I'm also struggling to use threads much in practice (finding it very hard to know when psychologically to start a thread rather than a reply, especially when reply is hardcoded muscle memory).

If replies are added to the thread, the user doesn't need to choose anymore. They can press reply if they think their message will be interesting for everyone in the room or thread if they don't, but whatever they do, their message will always be inside the thread; available for everyone who wants to follow that part of the conversation. And that will also benefit readers, because they'll know that any reply to that message can be found inside the thread - regardless of what button the person decided to press.

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

Element should show replies inside threads, but I'm not sure if any changes to the protocol are necessary for that.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

Start from the client flows for pressing Reply and for opening a thread, then compare how replies are currently represented in the room timeline and thread. Determine whether the desired behavior needs protocol changes. Done means a reply appears in both the room timeline and the thread rooted at the replied-to message.

Written by the indexing model from the issue text.