Insert a mention when clicking user's name in the timeline

[frebib]

Content

Clicking on a user's name in the timeline inserts a mention into the composer. Clicking on the avatar retains the old behaviour of opening the user details view.

Currently this doesn't work with the RTE due to some required changes in the RTE library that I haven't attempted yet.
One small "bug" is that the cursor is unconditionally jumped to the end of the text, not to the end of the mention (although the same happens when manually inserting a mention)

Motivation and context

By popular demand; also element-hq/element-x-ios#1527
This mirrors Element Android and Element Web behaviour.

Screenshots / GIFs

screen-20241112-124613.2.mp4

Tests

• Step 1
• Step 2
• Step ...

Tested devices

• Physical
• Emulator
• OS version(s): Android 15

Checklist

• Changes have been tested on an Android device or Android emulator with API 23
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request title will be used in the release note, it clearly define what will change for the user
• Pull request includes screenshots or videos if containing UI changes
• Pull request includes a sign off
• You've made a self review of your PR

[github-actions]

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[frebib]

This feels like a bit of a hack; not sure I like it. Maybe it would be better to move this to MarkdownTextEditorState and tidy it up a bit