Add ignore_trailing_commas option (#4609)

Added examples and modified the corresponding documents and unit tests.

Signed-off-by: chirsz-ever <chirsz-ever@outlook.com>
Co-authored-by: Niels Lohmann <niels.lohmann@gmail.com>

docs/mkdocs/docs/examples/trailing_commas.cpp

@@ -0,0 +1,37 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    std::string s = R"(
+    {
+        "planets": [
+            "Mercury",
+            "Venus",
+            "Earth",
+            "Mars",
+            "Jupiter",
+            "Uranus",
+            "Neptune",
+        ]
+    }
+    )";
+
+    try
+    {
+        json j = json::parse(s);
+    }
+    catch (json::exception& e)
+    {
+        std::cout << e.what() << std::endl;
+    }
+
+    json j = json::parse(s,
+                         /* callback */ nullptr,
+                         /* allow exceptions */ true,
+                         /* ignore_comments */ false,
+                         /* ignore_trailing_commas */ true);
+    std::cout << j.dump(2) << '\n';
+}