Rename 'project_core' crate to 'worktree', make it just about worktrees (#9189)

This is just a refactor. I noticed that we now have a `project_core`
crate, which mainly contains the `Worktree` type and its private
helpers, plus the project's settings.

In this PR, I've renamed that crate to `worktree` and did some minor
simplification to its module structure. I also extracted a new
`WorktreeSettings` settings type from the `ProjectSettings`, so that the
worktree settings could live in the worktree crate. This way, the crate
is now exclusively about worktree logic.

Release Notes:

- N/A

crates/file_finder/src/file_finder_tests.rs

@@ -4,7 +4,7 @@ use super::*;
 use editor::Editor;
 use gpui::{Entity, TestAppContext, VisualTestContext};
 use menu::{Confirm, SelectNext};
-use project::worktree::FS_WATCH_LATENCY;
+use project::FS_WATCH_LATENCY;
 use serde_json::json;
 use workspace::{AppState, Workspace};