Lay the groundwork for collaborating on assistant panel (#13991)

This pull request introduces collaboration for the assistant panel by
turning `Context` into a CRDT. `ContextStore` is responsible for sending
and applying operations, as well as synchronizing missed changes while
the connection was lost.

Contexts are shared on a per-project basis, and only the host can share
them for now. Shared contexts can be accessed via the `History` tab in
the assistant panel.

<img width="1819" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/c7ae46d2-cde3-4b03-b74a-6e9b1555c154">


Please note that this doesn't implement following yet, which is
scheduled for a subsequent pull request.

Release Notes:

- N/A

crates/language/src/buffer.rs

@@ -1903,6 +1903,10 @@ impl Buffer {
         self.deferred_ops.insert(deferred_ops);
     }
 
+    pub fn has_deferred_ops(&self) -> bool {
+        !self.deferred_ops.is_empty() || self.text.has_deferred_ops()
+    }
+
     fn can_apply_op(&self, operation: &Operation) -> bool {
         match operation {
             Operation::Buffer(_) => {