Form validation support

Related to [#21449](https://github.com/ampproject/amphtml/issues/21449): WorkerDOM currently doesn't support the [Constraint Validation API](https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/HTML5/Constraint_validation) which would be very useful for building multi-page, complex forms that are currently difficult to handle with `amp-bind` and `amp-form` alone.

Probably the easiest way to go would be implementing `checkValidity`, `reportValidity` and `setCustomValidity` as async methods evaluated on the main thread.

Start by reading the linked Constraint Validation API documentation and related issue #21449, then inspect WorkerDOM’s existing form API surface. Define support for async checkValidity, reportValidity, and setCustomValidity with evaluation on the main thread; done means these methods work for the form scenarios described.

Written by the indexing model from the issue text.