[RFC] Homebrew: move formula to neovim/homebrew-neovim

As proposal in https://github.com/neovim/neovim/issues/1401, move neovim.rb into [neovim/homebrew-neovim](https://github.com/neovim/homebrew-neovim) repo.

Associate PR: https://github.com/neovim/neovim/pull/1428