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

Applications/SystemMonitor/MemoryStatsWidget.cpp

@@ -98,7 +98,9 @@ void MemoryStatsWidget::refresh()
         ASSERT_NOT_REACHED();
 
     auto file_contents = proc_memstat->read_all();
-    auto json = JsonValue::from_string(file_contents).as_object();
+    auto json_result = JsonValue::from_string(file_contents);
+    ASSERT(json_result.has_value());
+    auto json = json_result.value().as_object();
 
     unsigned kmalloc_eternal_allocated = json.get("kmalloc_eternal_allocated").to_u32();
     (void)kmalloc_eternal_allocated;