theme: Split out theme_settings crate (#52569)

Self-Review Checklist:

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

Closes #ISSUE

Release Notes:

- N/A

crates/image_viewer/Cargo.toml

@@ -26,7 +26,7 @@ log.workspace = true
 project.workspace = true
 serde.workspace = true
 settings.workspace = true
-theme.workspace = true
+theme_settings.workspace = true
 ui.workspace = true
 util.workspace = true
 workspace.workspace = true