refactor: use new location for log files

[ojroques]

Hello,

Neovim 0.8 has changed the default location of log files to ~/.local/state/nvim: see neovim/neovim#14090 (comment). The issue is that the old log folder ~/.cache/nvim is not created upon installation of Neovim 0.8 anymore so Paq would crash trying to write into it.

This PR updates the log file location accordingly while keeping backward compatibility.

(I also changed the case of logfile for consistency, I can revert this change if needed).

[savq]

Neovim 0.8 has changed the default location of log files

Good to know. Thanks!

[sethidden]

If you're here because you're:

1. getting an error about something with "log" and "stdpath" realted to this line: https://github.com/savq/paq-nvim/blob/master/lua/paq.lua#L6
2. and you have nvim 0.8 installed

it's possible your version - despite being 0.8 - is not new enough. Make sure you have the latest nightly