assistant2: Add new conversation button, that also saves the current conversation (#11522)

This PR updates the new assistant with a button to start a new
conversation.

Clicking on it will reset the chat and put it into a fresh state.

The current conversation will be serialized and written to
`~/.config/zed/conversations`.

Release Notes:

- N/A

crates/assistant2/src/saved_conversation.rs

@@ -1,10 +1,27 @@
+use serde::{Deserialize, Serialize};
+
+use crate::MessageId;
+
+#[derive(Serialize, Deserialize)]
 pub struct SavedConversation {
+    /// The schema version of the conversation.
+    pub version: String,
     /// The title of the conversation, generated by the Assistant.
     pub title: String,
     pub messages: Vec<SavedMessage>,
 }
 
+#[derive(Serialize, Deserialize)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub enum SavedMessageRole {
+    User,
+    Assistant,
+}
+
+#[derive(Serialize, Deserialize)]
 pub struct SavedMessage {
+    pub id: MessageId,
+    pub role: SavedMessageRole,
     pub text: String,
 }
 
@@ -14,14 +31,17 @@ pub struct SavedMessage {
 pub fn placeholder_conversations() -> Vec<SavedConversation> {
     vec![
         SavedConversation {
+            version: "0.3.0".to_string(),
             title: "How to get a list of exported functions in an Erlang module".to_string(),
             messages: vec![],
         },
         SavedConversation {
+            version: "0.3.0".to_string(),
             title: "7 wonders of the ancient world".to_string(),
             messages: vec![],
         },
         SavedConversation {
+            version: "0.3.0".to_string(),
             title: "Size difference between u8 and a reference to u8 in Rust".to_string(),
             messages: vec![],
         },