AK: JsonParser improvements

- Parsing invalid JSON no longer asserts
    Instead of asserting when coming across malformed JSON,
    JsonParser::parse now returns an Optional<JsonValue>.
- Disallow trailing commas in JSON objects and arrays
- No longer parse 'undefined', as that is a purely JS thing
- No longer allow non-whitespace after anything consumed by the initial
  parse() call. Examples of things that were valid and no longer are:
    - undefineddfz
    - {"foo": 1}abcd
    - [1,2,3]4
- JsonObject.for_each_member now iterates in original insertion order

AK/JsonValue.cpp

@@ -75,7 +75,7 @@ void JsonValue::copy_from(const JsonValue& other)
 
 JsonValue::JsonValue(JsonValue&& other)
 {
-    m_type = exchange(other.m_type, Type::Undefined);
+    m_type = exchange(other.m_type, Type::Null);
     m_value.as_string = exchange(other.m_value.as_string, nullptr);
 }
 
@@ -83,7 +83,7 @@ JsonValue& JsonValue::operator=(JsonValue&& other)
 {
     if (this != &other) {
         clear();
-        m_type = exchange(other.m_type, Type::Undefined);
+        m_type = exchange(other.m_type, Type::Null);
         m_value.as_string = exchange(other.m_value.as_string, nullptr);
     }
     return *this;
@@ -247,11 +247,11 @@ void JsonValue::clear()
     default:
         break;
     }
-    m_type = Type::Undefined;
+    m_type = Type::Null;
     m_value.as_string = nullptr;
 }
 
-JsonValue JsonValue::from_string(const StringView& input)
+Optional<JsonValue> JsonValue::from_string(const StringView& input)
 {
     return JsonParser(input).parse();
 }