Support for questions in Element

@t3chguy suggested [this](https://github.com/vector-im/element-web/issues/21207) might more at home here.

### Your use case

#### What would you like to do?
I would like to be able to ask a question in a room, and for:
- the question message to be the start of a thread,
- for all responses to that question to be captured in the thread and alert me when they appear

I would like to be able to see all questions that have been asked of me, too. A question that has been asked of me is a question that:
- mentions me, or
- mentions `@room` (in a room I'm in, obvs)

Ideally I would also like:
- to be able to see somewhere all of the questions I have asked and all of the questions asked of me
- to be able to flag questions as resolved

#### Why would you like to do it?
Multitasking + Element is suboptimal. It is easy to lose answers to questions in the flow of scrollback. It is easy to forget a question that has been asked of you unless you address it immediately or store it in an external todo.

I want to fire and forget questions, and I want to be able to see all the questions (asked by and of me) to see where I need to reengage.

#### How would you like to achieve it?
We could probably construct it from our existing notifications and threading pieces. Maybe a

### Have you considered any alternatives?

Message starring does some of the work here, but without the power of notifying you when a starred message has had some further interaction, plus you have to _see_ it to star it.

### Additional context

Once you're flagging questions/answers it turns message history into an organically evolving knowledge base.

Start with the linked Element issue 21207 and review the existing notifications and threading pieces referenced in this issue. The work is complete when questions can begin threads, responses notify the asker, questions addressed to or asked by a user can be found, and resolution can be tracked.

Written by the indexing model from the issue text.