add missing methods to HTMLInputElement

This commit adds the following methods to the HTMLInputElement:

- CheckValidity
- SetCustomValidity
- Select
- SetSelectionRange
- StepDown
- StepUp

We explicitly do not add SetRangeText here, because its API and
functionality are confusing, would require adding a new set of constants
and because we'd rather wait until someone actually needs that method.
fix struct tags for ValidityState