Improve spine runtime

[genment]

This PR mainly updates the JSON parser, making it about 2x faster than the old version. I decided not to use System.Text.Json (STJ) because, while it’s a bit faster, the improvement isn’t significant enough to justify the big changes we'd need to make in the deserialization logic.

Benchmark:

Method	Mean	Error	StdDev	Ratio	Allocated
Old_2_1_25	10.369 ms	0.1986 ms	0.5634 ms	1.00	10.94 MB
Original_4_2	7.542 ms	0.1502 ms	0.1730 ms	0.73	6.01 MB
My_Updated_4_2	4.494 ms	0.0186 ms	0.0155 ms	0.43	4.61 MB
STJ	3.972 ms	0.0232 ms	0.0217 ms	0.39	1.63 MB

[lastbattle]

I'd typically not make any changes to the default spine-runtime library but its all great here.
Merged, thanks as always! 👍🏻