UTF-8 Support

[nervo]

Right now, utf-8 support is broken, because of golang behavior when looping with a range on string in ast readNode function

go-yaml/ast/ast.go

Line 269 in e4f32a2

for idx, b := range s[readLen : readLen+size] {

In this case, range loop over string runes and not string bytes. As a consequence, the resulting buffer bytes does not correspond to the string bytes themselves.

See the results on https://go.dev/play/p/5pWTN0y4vUj

Fortunately, a simple byte casting fix the issue :)

	for idx, b := range []byte(s[readLen : readLen+size]) {
		p[idx] = byte(b)
	}

A test has been provided with obscure utf-8 characters.

[goccy]

Thank you for the great PR !!! LGTM !