Allow exporting a range of message numbers

[inducer]

This is intended to help with #20445, once a chat history has gotten large enough to prevent it from being exported in one piece.

This works for my personal needs from a very quick bit of testing. Some work, particularly on tests, is needed. I may be able to do that, but I would first like to hear from the team on whether this direction is even considered desirable. I'm also open to feedback on how the range to be captured is described. The present "number/number to beginning" was driven mainly by least-edit-distance at first, but personally I'm finding it reasonably intuitive/usable.

I should add: I know next to nothing about React. This change is almost entirely monkey-see-monkey-do. I'm actually kind of surprised how far I was able to get with that approach. 🙂 So help/advice from someone more experience would definitely be more than welcome.

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)