zed: Add ability to restore last session w/ multiple windows (#14965)

This adds the ability for Zed to restore multiple windows after
restarting. It's now the default behavior.

Release Notes:

- Added ability to restore all windows that were open when Zed was quit.
Previously only the last used workspace was restored. This is now the
default behavior. To get back the old behavior, add the following to
your settings: `{"restore_on_startup": "last_workspace"}` (Part of
[#4985](https://github.com/zed-industries/zed/issues/4985) and
[#4683](https://github.com/zed-industries/zed/issues/4683))

Demo:



https://github.com/user-attachments/assets/57a375ec-0c6a-4724-97c4-3fea8f18bc2d

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/session/src/session.rs

@@ -0,0 +1,44 @@
+use db::kvp::KEY_VALUE_STORE;
+use util::ResultExt;
+use uuid::Uuid;
+
+#[derive(Clone, Debug)]
+pub struct Session {
+    session_id: String,
+    old_session_id: Option<String>,
+}
+
+impl Session {
+    pub async fn new() -> Self {
+        let key_name = "session_id".to_string();
+
+        let old_session_id = KEY_VALUE_STORE.read_kvp(&key_name).ok().flatten();
+
+        let session_id = Uuid::new_v4().to_string();
+
+        KEY_VALUE_STORE
+            .write_kvp(key_name, session_id.clone())
+            .await
+            .log_err();
+
+        Self {
+            session_id,
+            old_session_id,
+        }
+    }
+
+    #[cfg(any(test, feature = "test-support"))]
+    pub fn test() -> Self {
+        Self {
+            session_id: Uuid::new_v4().to_string(),
+            old_session_id: None,
+        }
+    }
+
+    pub fn id(&self) -> &str {
+        &self.session_id
+    }
+    pub fn last_session_id(&self) -> Option<&str> {
+        self.old_session_id.as_deref()
+    }
+}