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/worktree/src/ignore.rs

@@ -0,0 +1,53 @@
+use ignore::gitignore::Gitignore;
+use std::{ffi::OsStr, path::Path, sync::Arc};
+
+pub enum IgnoreStack {
+    None,
+    Some {
+        abs_base_path: Arc<Path>,
+        ignore: Arc<Gitignore>,
+        parent: Arc<IgnoreStack>,
+    },
+    All,
+}
+
+impl IgnoreStack {
+    pub fn none() -> Arc<Self> {
+        Arc::new(Self::None)
+    }
+
+    pub fn all() -> Arc<Self> {
+        Arc::new(Self::All)
+    }
+
+    pub fn append(self: Arc<Self>, abs_base_path: Arc<Path>, ignore: Arc<Gitignore>) -> Arc<Self> {
+        match self.as_ref() {
+            IgnoreStack::All => self,
+            _ => Arc::new(Self::Some {
+                abs_base_path,
+                ignore,
+                parent: self,
+            }),
+        }
+    }
+
+    pub fn is_abs_path_ignored(&self, abs_path: &Path, is_dir: bool) -> bool {
+        if is_dir && abs_path.file_name() == Some(OsStr::new(".git")) {
+            return true;
+        }
+
+        match self {
+            Self::None => false,
+            Self::All => true,
+            Self::Some {
+                abs_base_path,
+                ignore,
+                parent: prev,
+            } => match ignore.matched(abs_path.strip_prefix(abs_base_path).unwrap(), is_dir) {
+                ignore::Match::None => prev.is_abs_path_ignored(abs_path, is_dir),
+                ignore::Match::Ignore(_) => true,
+                ignore::Match::Whitelist(_) => false,
+            },
+        }
+    }
+}