This change allows using simple navigation keys (PageDown / PageUp /
Ctrl-Home / Ctrl-End) when the message editor is focused, but only if
the cursor is at the beginning/end of the message, where pressing these
keys would normally result in no-op.

One important corollary is that when the cursor is in an empty message,
navigation keys scroll the thread.

We already have this behavior for Up/Down and this change just expands
it for other navigation keys.

Demo:


[Demo](https://github.com/user-attachments/assets/ff540c8c-a223-417b-b16a-b0d08599b1ae)



Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable


Release Notes:

- N/A