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()`

Prefer to declare variables with correct type instead of explicit casts
wherever possible.

* Add optional second table argument to vim.json.decode which takes
  a table 'luanil' which can include the 'object' and/or 'array' keys. These
  options use luanil when converting NULL in json objects and arrays
  respectively. The default behavior matches the original lua-cjson.
* Remove recursive_convert_NIL function from rpc.lua, use
  vim.json.decode with luanil = { object = true } instead. This removes a hotpath
  in the json deserialization pipeline by dropping keys with json NULL
  values throughout the deserialized table.

* add vim.json.encode and vim.json.decode
* use vim.NIL instead of cjson.null
* resolve strict-prototypes warnings

* The following benchmark shows an approximately 2.5x (750 ms vs 300 ms) improvement in deserialization performance over
  vim.fn.json_decode on a medium package.json

  ```lua
  local uv = vim.loop
  local function readfile(path)
    return
  end

  local json_url = "https://raw.githubusercontent.com/rust-analyzer/rust-analyzer/b24c8d5c89ee93d1172b4127564f5da3b0c88dad/editors/code/package.json"
  io.popen(string.format('curl -v -f -L -O %q &> /dev/null', json_url))

  local json_string = io.open('package.json'):read '*a'

  uv.update_time()
  local start = uv.hrtime()

  for i = 1,1000 do
    vim.fn.json_decode(json_string)
  end

  uv.update_time()
  print(string.format("Deserialization time vim.fn.json_decode: %s ms", (uv.hrtime() - start) * (1e-6)))

  uv.update_time()
  local start = uv.hrtime()

  for i = 1,1000 do
    vim.json.decode(json_string)
  end
  uv.update_time()
  print(string.format("Deserialization time vim.json.decode: %s ms", (uv.hrtime() - start) * (1e-6)))
  ```

Co-Authored-By: Björn Linse <bjorn.linse@gmail.com>

Derived from the openresty lua-cjson fork at commit https://github.com/openresty/lua-cjson/commit/3d93d297092172eac3d52a1b3b6c1d479da5341a