feat: allow user to add custom prompts csv file to enhance `ChatGPTActAs` command

[csyJoy]

sometimes prompts in awesome-chatgpt-prompts are not enough, this feature allows user to add custom prompts csv file, csv file path is indecated by option local_chat_gpt_prompts_path which is nil by default.

It's my first PR to a neovim plugin, there may be some flaws in the PR, please let me know if it is. THX : )

[kohane27]

https://github.com/jackMort/ChatGPT.nvim/issues/52#issuecomment-1857966411