yaml.v3 encoding support for json.Number

[mihaitodor]

I noticed that #414 introduced encoding support for json.Number in yaml.v2. Is there any plan to port this feature to yaml.v3? Happy to put together a PR if that helps.

The following code:

package main

import (
	"encoding/json"
	"fmt"

	"gopkg.in/yaml.v3"
)

func main() {
	str := `{"foo": 3}`
	var res map[string]json.Number
	if err := json.Unmarshal([]byte(str), &res); err != nil {
		panic(err)
	}

	out, err := yaml.Marshal(res)
	if err != nil {
		panic(err)
	}

	fmt.Println(string(out))
}

prints foo: "3", but if I switch to yaml.v2, then it prints foo: 3.