x.json2: make encode number cJson compatible

[kbkpbot]

fix #22363

import json
import x.json2

pub struct Data {
	float64 f64
}

fn main() {
	data := Data{1.0}

	info_encoded := json.encode(data)
	println(info_encoded)
	info2_encoded := json2.encode(data)
	println(info2_encoded)
	assert info2_encoded == '{"float64":1}'  // not '{"float64":1.0}' anymore
}

[spytheman]

I think that can be done with return '${value:.15f}', but I have not tried.

[spytheman]

Excellent work.