Use simdjson in JSONParser
JSONParser predated the inclusion of simdjson, we were using the V8 JSON parser for convenience but it requires more than it needs to do the job (e.g. then the parser cannot be used to parse configs before V8 is initialized, then there's a chicken-and-egg problem if we want to use that to configure how to initialize V8).
| // We might want a lighter-weight JSON parser for this use case. But for now | |
| // using V8 is good enough. | |
| RAIIIsolateWithoutEntering isolate_; |
It would be simpler to just use simdjson to do this now that we have it.