assistant2: Allow removing individual context (#21868)

This PR adds the ability to remove individual pieces of context from the
message editor:

<img width="1159" alt="Screenshot 2024-12-11 at 12 38 45 PM"
src="https://github.com/user-attachments/assets/77d04272-f667-4ebb-a567-84b382afef3d"
/>

Release Notes:

- N/A

crates/assistant2/src/context.rs

@@ -1,8 +1,20 @@
 use gpui::SharedString;
+use serde::{Deserialize, Serialize};
+use util::post_inc;
+
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy, Serialize, Deserialize)]
+pub struct ContextId(pub(crate) usize);
+
+impl ContextId {
+    pub fn post_inc(&mut self) -> Self {
+        Self(post_inc(&mut self.0))
+    }
+}
 
 /// Some context attached to a message in a thread.
 #[derive(Debug, Clone)]
 pub struct Context {
+    pub id: ContextId,
     pub name: SharedString,
     pub kind: ContextKind,
     pub text: SharedString,