This PR revamps our feature flag system, to enable richer iteration.
Feature flags can now:
- Support enum values, for richer configuration
- Be manually set via the settings file
- Be manually set via the settings UI
This PR also adds a feature flag to demonstrate this behavior, a
`agent-thread-worktree-label`, which controls which how the worktree tag
UI displays.
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:
- N/A
