Fix JsonEncode treating tables with index 0 as arrays (#454)

## Summary
- Fix `Util.JsonEncode` incorrectly encoding Lua tables with index 0 as
JSON arrays
- Added `IsLuaArray()` helper function that validates all keys are
integers >= 1
- Tables with non-standard indices (like 0) are now correctly encoded as
JSON objects

  Closes #348

  ## Before
  ```lua
  Util.JsonEncode({[0]="a", [1]=true, [2]=0.1})
  -- Output: ["a",true,0.1]  (incorrect - data loss)
  ```
  ## After
  ```lua
  Util.JsonEncode({[0]="a", [1]=true, [2]=0.1})
  -- Output: {"0":"a","1":true,"2":0.1}  (correct)
  ```

src/LuaAPI.cpp

@@ -693,6 +693,22 @@ std::string LuaAPI::FS::ConcatPaths(sol::variadic_args Args) {
     return Result;
 }
 
+static bool IsLuaArray(const sol::table& table) {
+    for (const auto& pair : table) {
+        if (pair.first.get_type() != sol::type::number) {
+            return false;
+        }
+        if (!pair.first.is<int>()) {
+            return false;
+        }
+        int key = pair.first.as<int>();
+        if (key < 1) {
+            return false;
+        }
+    }
+    return true;
+}
+
 static void JsonEncodeRecursive(nlohmann::json& json, const sol::object& left, const sol::object& right, bool is_array, size_t depth = 0) {
     if (depth > 100) {
         beammp_lua_error("json serialize will not go deeper than 100 nested tables, internal references assumed, aborted this path");
@@ -762,12 +778,7 @@ static void JsonEncodeRecursive(nlohmann::json& json, const sol::object& left, c
         if (right.as<sol::table>().empty()) {
             value = nlohmann::json::object();
         } else {
-            bool local_is_array = true;
-            for (const auto& pair : right.as<sol::table>()) {
-                if (pair.first.get_type() != sol::type::number) {
-                    local_is_array = false;
-                }
-            }
+            bool local_is_array = IsLuaArray(right.as<sol::table>());
             for (const auto& pair : right.as<sol::table>()) {
                 JsonEncodeRecursive(value, pair.first, pair.second, local_is_array, depth + 1);
             }
@@ -790,12 +801,7 @@ std::string LuaAPI::MP::JsonEncode(const sol::table& object) {
     if (object.as<sol::table>().empty()) {
         json = nlohmann::json::object();
     } else {
-        bool is_array = true;
-        for (const auto& pair : object.as<sol::table>()) {
-            if (pair.first.get_type() != sol::type::number) {
-                is_array = false;
-            }
-        }
+        bool is_array = IsLuaArray(object.as<sol::table>());
         for (const auto& entry : object) {
             JsonEncodeRecursive(json, entry.first, entry.second, is_array);
         }