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

Userland/lsirq.cpp

@@ -60,8 +60,9 @@ int main(int argc, char** argv)
 
     printf("%4s  %-10s\n", " ", "CPU0");
     auto file_contents = proc_interrupts->read_all();
-    auto json = JsonValue::from_string(file_contents).as_array();
-    json.for_each([](auto& value) {
+    auto json = JsonValue::from_string(file_contents);
+    ASSERT(json.has_value());
+    json.value().as_array().for_each([](auto& value) {
         auto handler = value.as_object();
         auto purpose = handler.get("purpose").to_string();
         auto interrupt = handler.get("interrupt_line").to_string();