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/zed/src/zed.rs

@@ -866,7 +866,7 @@ mod tests {
         VisualTestContext, WindowHandle,
     };
     use language::{LanguageMatcher, LanguageRegistry};
-    use project::{project_settings::ProjectSettings, Project, ProjectPath};
+    use project::{Project, ProjectPath, WorktreeSettings};
     use serde_json::json;
     use settings::{handle_settings_file_changes, watch_config_file, SettingsStore};
     use std::path::{Path, PathBuf};
@@ -1480,7 +1480,7 @@ mod tests {
         let app_state = init_test(cx);
         cx.update(|cx| {
             cx.update_global::<SettingsStore, _>(|store, cx| {
-                store.update_user_settings::<ProjectSettings>(cx, |project_settings| {
+                store.update_user_settings::<WorktreeSettings>(cx, |project_settings| {
                     project_settings.file_scan_exclusions =
                         Some(vec!["excluded_dir".to_string(), "**/.git".to_string()]);
                 });