feat(lua): expose lua-cjson as vim.json

* 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>

src/nvim/lua/executor.c

@@ -40,6 +40,7 @@
 #include "nvim/undo.h"
 #include "nvim/version.h"
 #include "nvim/vim.h"
+#include "cjson/lua_cjson.h"
 
 static int in_fast_callback = 0;
 
@@ -531,6 +532,9 @@ static int nlua_state_init(lua_State *const lstate) FUNC_ATTR_NONNULL_ALL
   lua_pushcfunction(lstate, &nlua_xdl_diff);
   lua_setfield(lstate, -2, "diff");
 
+  lua_cjson_new(lstate);
+  lua_setfield(lstate, -2, "json");
+
   lua_setglobal(lstate, "vim");
 
   {