editors/neovim: Update to 0.12.0

This is a very big release--one of the biggest Neovim has ever
had.

Changes: https://github.com/neovim/neovim/blob/v0.12.0/runtime/doc/news.txt

In addition to major LSP improvements, there is a new UI API that will
allow nvim and plugins to engage messages, windows, progress bars, etc.
for a consistent UX. The new vim.pack plugin manager utilizes it to
provide a built-in alternative to lazy.nvim (this one is big!).