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

Libraries/LibCore/ProcessStatisticsReader.cpp

@@ -48,7 +48,8 @@ HashMap<pid_t, Core::ProcessStatistics> ProcessStatisticsReader::get_all()
 
     auto file_contents = file->read_all();
     auto json = JsonValue::from_string(file_contents);
-    json.as_array().for_each([&](auto& value) {
+    ASSERT(json.has_value());
+    json.value().as_array().for_each([&](auto& value) {
         const JsonObject& process_object = value.as_object();
         Core::ProcessStatistics process;