Ref https://vimdoc.sourceforge.net/htmldoc/options.html#ex%3A

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [ ] 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

Closes N/A

Release Notes:

- Support `ex:` identifier in vim modelines.

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Co-authored-by: dino <dinojoaocosta@gmail.com>