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/worktree_settings.rs

@@ -0,0 +1,40 @@
+use gpui::AppContext;
+use schemars::JsonSchema;
+use serde::{Deserialize, Serialize};
+use settings::Settings;
+
+#[derive(Clone, Default, Serialize, Deserialize, JsonSchema)]
+pub struct WorktreeSettings {
+    /// Completely ignore files matching globs from `file_scan_exclusions`
+    ///
+    /// Default: [
+    ///   "**/.git",
+    ///   "**/.svn",
+    ///   "**/.hg",
+    ///   "**/CVS",
+    ///   "**/.DS_Store",
+    ///   "**/Thumbs.db",
+    ///   "**/.classpath",
+    ///   "**/.settings"
+    /// ]
+    #[serde(default)]
+    pub file_scan_exclusions: Option<Vec<String>>,
+
+    /// Treat the files matching these globs as `.env` files.
+    /// Default: [ "**/.env*" ]
+    pub private_files: Option<Vec<String>>,
+}
+
+impl Settings for WorktreeSettings {
+    const KEY: Option<&'static str> = None;
+
+    type FileContent = Self;
+
+    fn load(
+        default_value: &Self::FileContent,
+        user_values: &[&Self::FileContent],
+        _: &mut AppContext,
+    ) -> anyhow::Result<Self> {
+        Self::load_via_json_merge(default_value, user_values)
+    }
+}