add json.h (#1716)

* add json.h

* Update xmake.lua

packages/j/json.h/xmake.lua

@@ -0,0 +1,15 @@
+package("json.h")
+    set_kind("library", {headeronly = true})
+    set_homepage("https://github.com/sheredom/json.h")
+    set_description("single header json parser for C and C++")
+
+    add_urls("https://github.com/sheredom/json.h.git")
+    add_versions("2022.11.27", "06aa5782d650e7b46c6444c2d0a048c0a1b3a072")
+
+    on_install(function (package)
+        os.cp("*.h", package:installdir("include"))
+    end)
+
+    on_test(function (package)
+        assert(package:has_cfuncs("json_parse", {includes = "json.h"}))
+    end)