Allow read markers to go backwards

**Suggestion**
In order for front-ends to be able to implement "Mark as Unread from this message" (like it's possible in Skype), Matrix needs to allow the read marker to go backwards.

See issue in Element: https://github.com/element-hq/element-meta/issues/347

See this comment (and the following one): https://github.com/element-hq/element-meta/issues/347#issuecomment-2199591748

Issue raised for Element for this specific feature: https://github.com/element-hq/element-meta/issues/2465

Thank you!

Start by reading the Matrix read-marker specification and the linked Element issues, especially the discussion around issue 347 and the feature request in issue 2465. Define the protocol changes needed to permit moving a read marker backwards and the compatibility and completion criteria before implementation.

Written by the indexing model from the issue text.