Better model selection support

Author: Niek

src/lib/Chat.svelte

@@ -165,7 +165,6 @@
     let response: Response;
     try {
       const request: Request = {
-        model: "gpt-3.5-turbo",
         // Submit only the role and content of the messages, provide the previous messages as well for context
         messages: messages
           .map((message): Message => {

src/lib/Types.svelte

@@ -18,7 +18,7 @@
   };
 
   export type Request = {
-    model: "gpt-3.5-turbo" | "gpt-3.5-turbo-0301";
+    model?: Model;
     messages: Message[];
     temperature?: number;
     top_p?: number;