common: add json.h abstraction (#27511)

* add common/json

* migrate common

* adapt jinja

* migrate server

* big wip

* migrate tests

* wip

* revert some excessive changes

* wip

* wip 2

* revert redundant changes

* fix server crash

* various fixes

* fix ci

* harden a bit

* clean up

* rm json-shim

* add some comments

* rm redundant decl

tests/test-jinja.cpp

@@ -3,7 +3,7 @@
 #include <random>
 #include <cstdlib>
 
-#include <nlohmann/json.hpp>
+#include "json.h"
 #include "subproc.h"
 
 #include "jinja/runtime.h"
@@ -14,7 +14,7 @@
 
 #include "testing.h"
 
-using json = nlohmann::ordered_json;
+using json = common_json;
 
 static void test_template(testing & t, const std::string & name, const std::string & tmpl, const json & vars, const std::string & expect);
 
@@ -240,7 +240,7 @@ static void test_conditionals(testing & t) {
 
     test_template(t, "is undefined key falsy",
         "{{ 'yes' if not y['x'] else 'no' }}",
-        {{"y", {{}}}},
+        {{"y", json::array({nullptr})}},
         "yes"
     );
 
@@ -282,7 +282,7 @@ static void test_conditionals(testing & t) {
 
     test_template(t, "is non-empty object truthy",
         "{{ 'yes' if y else 'no' }}",
-        {{"y", {"x", false}}},
+        {{"y", json::array({"x", false})}},
         "yes"
     );