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/JsonParser.h

@@ -40,23 +40,24 @@ public:
     {
     }
 
-    JsonValue parse();
+    Optional<JsonValue> parse();
 
 private:
+    Optional<JsonValue> parse_helper();
+
     char peek() const;
     char consume();
     void consume_whitespace();
-    void consume_specific(char expected_ch);
-    void consume_string(const char*);
+    bool consume_specific(char expected_ch);
+    bool consume_string(const char*);
     String consume_quoted_string();
-    JsonArray parse_array();
-    JsonObject parse_object();
-    JsonValue parse_number();
-    JsonValue parse_string();
-    JsonValue parse_false();
-    JsonValue parse_true();
-    JsonValue parse_null();
-    JsonValue parse_undefined();
+    Optional<JsonValue> parse_array();
+    Optional<JsonValue> parse_object();
+    Optional<JsonValue> parse_number();
+    Optional<JsonValue> parse_string();
+    Optional<JsonValue> parse_false();
+    Optional<JsonValue> parse_true();
+    Optional<JsonValue> parse_null();
 
     template<typename C>
     void consume_while(C);