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>

1 files changed, 10 insertions, 0 deletions

diff --git a/src/cjson/lua_cjson.h b/src/cjson/lua_cjson.h
new file mode 100644
index 0000000000..3f70b679be
--- /dev/null
+++ b/src/cjson/lua_cjson.h
@@ -0,0 +1,10 @@
+#ifndef CJSON_LUACJSON_H
+#define CJSON_LUACJSON_H
+
+#include "lua.h"
+
+int lua_cjson_new(lua_State *l);
+int luaopen_cjson(lua_State *l);
+int luaopen_cjson_safe(lua_State *l);
+
+#endif  // CJSON_LUACJSON_H