Implement ExtSimdJsonDecoder

[JakubOnderka]

Hello,

I am an author of forked simdjson_php extension for PHP, that can be used as another decoder backend (ExtSimdJsonDecoder) for your library.

Decoding JSON with simdjson is 3 times faster than ExtJson and also uses less memory.

[halaxa]

Hi, thank you very much for this. Looks interesting. I can't find ExtSimdJsonDecoder in your repo. Can you elaborate on how could it work together?