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_settings.rs

@@ -0,0 +1,63 @@
+use collections::HashMap;
+use gpui::AppContext;
+use schemars::JsonSchema;
+use serde::{Deserialize, Serialize};
+use settings::Settings;
+use std::sync::Arc;
+
+#[derive(Clone, Default, Serialize, Deserialize, JsonSchema)]
+pub struct ProjectSettings {
+    /// Configuration for language servers.
+    ///
+    /// The following settings can be overridden for specific language servers:
+    /// - initialization_options
+    /// To override settings for a language, add an entry for that language server's
+    /// name to the lsp value.
+    /// Default: null
+    #[serde(default)]
+    pub lsp: HashMap<Arc<str>, LspSettings>,
+
+    /// Configuration for Git-related features
+    #[serde(default)]
+    pub git: GitSettings,
+}
+
+#[derive(Copy, Clone, Debug, Default, Serialize, Deserialize, JsonSchema)]
+pub struct GitSettings {
+    /// Whether or not to show the git gutter.
+    ///
+    /// Default: tracked_files
+    pub git_gutter: Option<GitGutterSetting>,
+    pub gutter_debounce: Option<u64>,
+}
+
+#[derive(Clone, Copy, Debug, Default, Serialize, Deserialize, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+pub enum GitGutterSetting {
+    /// Show git gutter in tracked files.
+    #[default]
+    TrackedFiles,
+    /// Hide git gutter
+    Hide,
+}
+
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq, Eq, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+pub struct LspSettings {
+    pub initialization_options: Option<serde_json::Value>,
+    pub settings: Option<serde_json::Value>,
+}
+
+impl Settings for ProjectSettings {
+    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)
+    }
+}