feat: model configuration to support different models (#183)

Co-authored-by: hiroki osame <hiroki.osame@gmail.com>

src/utils/config.ts

@@ -2,6 +2,7 @@ import fs from 'fs/promises';
 import path from 'path';
 import os from 'os';
 import ini from 'ini';
+import type { TiktokenModel } from '@dqbd/tiktoken';
 import { fileExists } from './fs.js';
 import { KnownError } from './error.js';
 
@@ -60,6 +61,13 @@ const configParsers = {
 
 		return url;
 	},
+	model(model?: string) {
+		if (!model || model.length === 0) {
+			return 'gpt-3.5-turbo';
+		}
+
+		return model as TiktokenModel;
+	},
 } as const;
 
 type ConfigKeys = keyof typeof configParsers;