chore: moving from im_end to completion params

As we allow the constructor to define completionParams, it would be better to allow devs using the library to use the specified stop token.
As of right now, the library allows such stop to be passed but it isn't considered in the class itself.

src/chatgpt-api.ts

@@ -307,7 +307,7 @@ Current date: ${currentDate}\n\n`
 
     const maxNumTokens = this._maxModelTokens - this._maxResponseTokens
     let { parentMessageId } = opts
-    let nextPromptBody = `${this._userLabel}:\n\n${message}<|im_end|>`
+    let nextPromptBody = `${this._userLabel}:\n\n${message}${this._completionParams.stop}`
     let promptBody = ''
     let prompt: string
     let numTokens: number
@@ -343,7 +343,7 @@ Current date: ${currentDate}\n\n`
         parentMessageRole === 'user' ? this._userLabel : this._assistantLabel
 
       // TODO: differentiate between assistant and user messages
-      const parentMessageString = `${parentMessageRoleDesc}:\n\n${parentMessage.text}<|im_end|>\n\n`
+      const parentMessageString = `${parentMessageRoleDesc}:\n\n${parentMessage.text}${this._completionParams.stop}\n\n`
       nextPromptBody = `${parentMessageString}${promptBody}`
       parentMessageId = parentMessage.parentMessageId
     } while (true)