Problem:
- `vim.json` exposes various global options which:
  - affect all Nvim Lua plugins (especially the LSP client)
  - are undocumented and untested
  - can cause confusing problems such as: https://github.com/codota/tabnine-nvim/commit/cc76ae3abe2f129d44b5a8edee2529e0ee0dcf69
- `vim.json` exposes redundant mechanisms:
  - `vim.json.null` is redundant with `vim.NIL`.
  - `array_mt` is redundant because Nvim uses a metatable
    (`vim.empty_dict()`) for empty dict instead, which `vim.json` is
    configured to use by default (see `as_empty_dict`).
    Example:
    ```
    :lua vim.print(vim.json.decode('{"bar":[],"foo":{}}'))
    --> { bar = {},  foo = vim.empty_dict() }
    ```
    Thus we don't need to also decorate empty arrays with `array_mt`.

Solution:
Remove the functions from the public vim.json interface.
Comment-out the implementation code to minimize drift from upstream.

TODO:
- Expose the options as arguments to `vim.json.new()`