Refactor new_session method for better API key handling

[Toukaya]

• Moved the logic for determining the API key environment variable to a new static method get_api_key_env_var.
• Updated new_session method to use get_api_key_env_var for retrieving the correct API key environment variable based on the model name.
• Set the default model to gpt-3.5-turbo using kwargs.setdefault for cleaner code.
• Added support for setting the API URL if base_url is provided in kwargs.
• Ensured that the API key is validated before creating a new ChatGPTSession.

Therefore, it is now possible to temporarily support more large language models, such as QWEN, MoonShot, etc., through the setting of the base URL.

like:

qwen_ai = AIChat(
    console=False, 
    base_url="https://dashscope.aliyuncs.com/compatible-mode/v1", 
    model="qwen-plus"
)