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/git_ui/src/commit_tooltip.rs

@@ -12,7 +12,7 @@ use markdown::{Markdown, MarkdownElement};
 use project::git_store::Repository;
 use settings::Settings;
 use std::hash::Hash;
-use theme::ThemeSettings;
+use theme_settings::ThemeSettings;
 use time::{OffsetDateTime, UtcOffset};
 use ui::{Avatar, CopyButton, Divider, prelude::*, tooltip_container};
 use workspace::Workspace;