Closes #55481

Adds Vim-mode access to the existing Helix jump-to-word overlay via `g
z`. We use `g z` because it is currently unassigned in Vim mode, while
`g w` is already used for rewrap.

Most of the implementation lives in `helix.rs` because the existing jump
overlay, label generation, and Helix/Vim modal behavior are currently
intertwined there. This keeps the change small and reuses the existing
navigation overlay logic instead of doing a broader refactor.

In Vim normal mode, jump labels behave like a cursor motion: selecting a
label moves the cursor to the start of the target word without selecting
it. In Vim visual mode, jump labels extend the selection like a Vim
word-start motion, preserving Vim’s inclusive visual-selection behavior.

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)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Added Vim-mode jump-to-word navigation on `g z`.