Feature request: Unmarshal error with source position

[rhysd]

Hi,

I'm using this library to develop actionlint. Since it is a linter, I want to know the exact source position of unmarshal error so that it can report exact error position with helpful source snippet.

Currently go-yaml/yaml's unmarshal error does not have source position information though line number is embedded in an error message.

Example usage is as follows:

package main

import (
	"fmt"

	"gopkg.in/yaml.v3"
)

func main() {
	in := "#\n - - QI\xd7"

	var n yaml.Node
	if err := yaml.Unmarshal([]byte(in), &n); err != nil {
		err, _ := err.(*yaml.TypeError)
		for _, err := range err.Errors {
			// I want the err.Line and err.Column fields
			fmt.Fprintf("yaml parse failure at line:%d col:%d: %s\n", err.Line, err.Column, err.Message)
		}
		fmt.Fprintf("%d errors found while parsing yaml file\n", len(err.Errors))
	}
}