Update (n)vim plugin manager installation section

[tuurep]

Once Neovim 0.12 gets rolled out across the board, would make sense for vim.pack to be the main example for installation on Neovim. Vim examples are fine, but would be good to have a separate section for Vim and Neovim.

Word it something like this:

## Installation

With [Vivify](https://github.com/jannis-baum/Vivify) installed in your PATH, you
can install this plugin just like any other Vim plugin.

### Vim

- with Vim8's [native package manager](https://vimhelp.org/repeat.txt.html#packages)
  by adding it to your `~/.vim/pack/plugin/start`
- with [vim-plug](https://github.com/junegunn/vim-plug):
  `Plug 'jannis-baum/vivify.vim'`

### Neovim

- with Neovim's [builtin plugin manager](https://neovim.io/doc/user/pack/#vim.pack):
  `vim.pack.add({ 'https://github.com/jannis-baum/vivify.vim' })`
- with [lazy.nvim](https://github.com/folke/lazy.nvim):
  `{ "jannis-baum/vivify.vim" }`