Buffer is not 'modifiable' error when submitting a new query while one is still executing.

[joeldrotleff]

With the main ChatGPT window open, while a query is being executed, submitting a new query leads to an error being printed over and over:

Error executing vim.schedule lua callback: ...e/nvim/lazy/ChatGPT.nvim/lua/chatgpt/flows/chat/base.lua:263: Buffer is not 'modifiable'
stack traceback:
[C]: in function 'nvim_buf_set_lines'
...e/nvim/lazy/ChatGPT.nvim/lua/chatgpt/flows/chat/base.lua:263: in function 'addAnswerPartial'
...e/nvim/lazy/ChatGPT.nvim/lua/chatgpt/flows/chat/base.lua:723: in function ''
vim/_editor.lua: in function <vim/_editor.lua:0>

With (apparently) no good way to quit - :q, , C-c, C-d...nothing seems to work.

Steps to reproduce:

1. Open ChatGPT plugin :ChatGPT
2. Request something that takes a while to complete, i.e. "Tell me a story." Wait for response to begin streaming in.
3. Type a new query, i.e. "Test" and hit Enter.
   Notice: Error is shown at bottom of screen, and can't be dismissed
   Expected: No error. I think a warning that says something like "You have a query running already. To cancel it, use C-x (the default cancel key command).

[joeldrotleff]

Here's a fix for it: #341