Add json_t to benchmark

[SenseOnline]

Since I was curious to know the overhead incurred from using glz::json_t, others may also be interested to know too.

MSVC
AMD Ryzen Threadripper 3990X 64-Core Processor 2.90 GHz

Library	Roundtrip Time (s)	Write (MB/s)	Read (MB/s)
Glaze	2.42	673	453
Glaze json_t	4.28	467	224
simdjson (on demand)	N/A	N/A	243
yyjson	2.23	551	630
daw_json_link	6.25	202	211
RapidJSON	7.29	240	141
Boost.JSON (direct)	10.28	95	196
json_struct	9.42	138	140
nlohmann	41.36	35	28

[stephenberry]

Thanks for the suggestion. This would be good to benchmark.