Problem:
The bundled `zip` plugin is implemented in Vimscript, making it harder to
maintain and build on with Nvim's Lua runtime infrastructure.

Solution:
Add an opt-out `zip.lua` browser backed by `nvim.dir`, and disable the legacy
Vimscript implementation by moving it to `pack/dist/opt/zip/`. Load it with
`:packadd zip`.

redux 24e00f2844bc

text-object-define is a pattern I found in tpope's plugins (e.g.
https://github.com/tpope/vim-jdaddy) which shows an elegant way to
define a text-object. (Any mistakes in the example are my fault.)

Problem:
Opening .tar.gz or .zip URLs shows raw binary instead of using the archive plugins.

Solution:
Similar to the original netrw implementation, the autocmd should detect
archive URLs, download them to a temp file and the open them with
tar/zip handlers already bundled as vim plugins.

Problem:
Non-trivial to write output of vim.net.request to buffer. Requires extra
code in plugin/net.lua which can't be reused by other plugin authors.

```
vim.net.request('https://neovim.io', {}, function(err, res)
  if not err then
    local buf = vim.api.nvim_create_buf(true, false)
    if res then
      local lines = vim.split(res.body, '\n', { plain = true })
      vim.api.nvim_buf_set_lines(buf, 0, -1, true, lines)
    end
  end
end)
```

Solution:
Accept an optional `outbuf` argument to indicate the buffer to write output
to, similar to `outpath`.

    vim.net.request('https://neovim.io', { outbuf = buf })

Other fixes / followups:
- Make plugin/net.lua smaller
- Return objection with close() method
- vim.net.request.Opts class
- vim.validate single calls
- Use (''):format(...) instead of `..`

Problem:
The "tohtml" plugin is loaded by default.

Solution:
- Move it to `pack/dist/opt/nvim.tohtml/`, it is an "opt-in" plugin now.
- Document guidelines.
- Also revert the `plugin/` locations of `spellfile.lua` and `net.lua`.
  That idea was not worth the trouble, it will be too much re-education
  for too little gain.