json: allow anon struct as decode type

[felipensp]

Fix #21703

import json

fn main() {
	json_text := '{ "a": "b" }'
	b := json.decode(struct {
		a string
	}, json_text)!.a
	assert dump(b) == 'b'
}

[JalonSolov]

Really should be improving x.json2 so json can be retired...

[Delta456]

It is better to support passing anonymous structs as parameters in general not just json.

[JalonSolov]

If they're going to be supported at all, then yes, they should be supported in general.

[felipensp]

But it's about anon struct declaration, not initialization.

[spytheman]

Excellent work.