feat: add Unwrap() []error to ParseError for errors.Is/As support

[Yanhu007]

Fixes #47

Problem

ParseError wraps multiple errors in its Errs field but does not implement the Unwrap() []error interface (Go 1.20+). This means errors.Is and errors.As cannot inspect the underlying errors.

Fix

Add the Unwrap method:

func (e ParseError) Unwrap() []error {
    return e.Errs
}

Testing

All existing tests pass.