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/project/src/project_tests.rs

@@ -14,6 +14,7 @@ use serde_json::json;
 use std::{os, task::Poll};
 use unindent::Unindent as _;
 use util::{assert_set_eq, paths::PathMatcher, test::temp_tree};
+use worktree::WorktreeModelHandle as _;
 
 #[gpui::test]
 async fn test_block_via_channel(cx: &mut gpui::TestAppContext) {