[Feature] Use nix for conform.vim and nvim-lint

[polnio]

Today, when using lsp with nixvim, it install the language server automatically, and we can customize the package too. I think it's a cool feature, and I wonder if it is possible to do pretty much the same thing for conform.nvim and nvim-lint. At the moment we have to use a string for the plugin, and to not forget to add the package by hand.

Supported formatters and linters are listed on their README.md:

• conform.nvim
• nvim-lint

[khaneliman]

Similar to #1141