Set buftype=nowrite for otter buffers

[yongrenjie]

Hello!

I've been experiencing issue #7, and the fact that I can't consistently reproduce it either is quite annoying. However, I think this PR should fix it! :h buftype describes the nowrite option:

The buffer is not to be written to disk, ":w" doesn't work (":w filename" does work though).
The buffer is never considered to be |'modified'|. There is no warning when the changes will be lost, for example when you quit Vim.

An added bonus is that it's no longer necessary to delete these files upon exiting, because it is not possible to save it.

So far it's working fine for me and I haven't run into the issue again, but I'll attach the caveat that I don't use Quarto regularly enough to be fully confident in saying that it'll never happen. 😄

[jmbuhr]

This is amazing, thank you very much! I had previously experimented with nowrite and couldn't get it to work because it would interfer with set_lines. I probably set the option the wrong way.

No errors so far on my side as well.

Fixes #7