Beta testing `nvim-0.12` reference config

[echasnovski]

Problem description

Reference config nvim-0.12 (for Neovim>=0.12) is currently in beta-testing. The general structure and overall design are stable and should work fine.

However, there might be some minor issues that are specific for nvim-0.12 config in particular that might improved or clarified. Please feel free to report them here.