Add Is() methods for error types.

[Eun]

Description

This pull requests adds the Is(err error) bool method to the errors.

Problem

Errors are hard to distinguish. It is not possible to follow custom logic in case of error.
Take the following code as an example:

func getName(node *dasel.Node) (string, error) {
	result, err := node.Query(".Name")
	if err != nil {
		if errors.Is(err, &dasel.ValueNotFound{}) {
			return "", errors.New("name not found")
		}
		return "", err
	}
	return result.String(), nil
}

Solution

Implement the Is method so the errors.Is() function can be used.

[TomWright]

Thanks for the contribution!