Add Builder feature

[m-mizutani]

This PR adds goerr.NewBuilder() to create an error with pre-defined contextual variables. It is useful when you want to create an error with the same contextual variables in multiple places.

type object struct {
	id    string
	color string
}

func (o *object) Validate() error {
	eb := goerr.NewBuilder().With("id", o.id)

	if o.color == "" {
		return eb.New("color is empty")
	}

	return nil
}

func main() {
	obj := &object{id: "object-1"}

	if err := obj.Validate(); err != nil {
		slog.Default().Error("Validation error", "err", err)
	}
}

Output:

2024/10/19 14:19:54 ERROR Validation error err.message="color is empty" err.values.id=object-1 (snip)