Remove fallback replies & implement intentional mentions (#2138)

Remove fallback replies & implement intentional mentions (#2138)

* Remove reply fallbacks & add m.mentions

(WIP) the typing on line 301 and 303 needs fixing but apart from that this is mint

* Less jank typing

* Mention the reply author in m.mentions

* Improve typing

* Fix typing in m.mentions finder

* Correctly iterate through editor children, properly handle @room, ...

..., don't mention the reply author when the reply author is ourself, don't add own user IDs when mentioning intentionally

* Formatting

* Add intentional mentions to edited messages

* refactor reusable code and fix todo

* parse mentions from all nodes

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Co-authored-by: Ajay Bura <32841439+ajbura@users.noreply.github.com>