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/editor/src/editor.rs

@@ -3245,7 +3245,7 @@ impl Editor {
             .filter(|(_, excerpt_visible_range, _)| !excerpt_visible_range.is_empty())
             .filter_map(|(buffer_handle, excerpt_visible_range, excerpt_id)| {
                 let buffer = buffer_handle.read(cx);
-                let buffer_file = project::worktree::File::from_dyn(buffer.file())?;
+                let buffer_file = project::File::from_dyn(buffer.file())?;
                 let buffer_worktree = project.worktree_for_id(buffer_file.worktree_id(cx), cx)?;
                 let worktree_entry = buffer_worktree
                     .read(cx)